who recently saw a provider I plan to see to ask about STD issues subsequent to their visit?  I saw a post recently that referenced back channel as a way to determine the relative safety of seeing a particular provider (I know nothing is 100% safe).  Is PM what was meant by "back channel" or is there someplace else I should be researching

There is nothing wrong with asking others who have seen the same provider as you. Is there a reason why you would be concerned about STD's? If you have something it would be best to contact the provider first, one for discretion and 2 because he may not have it, she may not have it and you would have ruined the reputation of the provider for no reason.  

I wouldn't expect a response, if I were you.  It's the sort of question that comes off as trolling, or just plain too noobish, paranoid, whatever.
  Yes, PMing is considered "back channel" correspondence.  

There may be useful information to be had on the RO board, as well as what's exchanged among private groups.  But it sounds like you're not there yet, so those don't really apply.  

 
Just a little gentle advice: if you're so concerned that you feel you need a post-date "clean bill of health" from a provider's other clients...... this activity may not be for you.  

If you PM a prior reviewer and you do so using an alias I wouldn't expect a response.  

If you have no reviews, again I wouldn't expect a response.

Lastly, not every person has VIP, so you may not get a response.

It is fine to PM a reviewer about a provider they reviewed that you plan on seeing.  They may or may not respond for various reasons (apathy, fear of LEO, non-VIP, won't respond to PM from alias, etc.).  

However, that particular question seems particularly unlikely to elicit any useful information.  The reviewer may find it insulting or just bizarre.  Plus, many STDs won't even show up on a test until 6 months or so after initial infection.  What are the chances that the reviewer had sex with the provider you're interested in, got thoroughly tested at least 6 months later, and is willing to truthfully share the results of their highly personal and private STD tests with you?  If they told you they were clean, could you really believe them?  Even if they were willing to tell you they had picked up an STD, could you really narrow it down to that provider?  Who else has the reviewer been with that could have given them an STD?  Do you really think you'll get honest answers to questions like that?  Can you be sure?  How much stock are you willing to put in the honesty of a random reviewer?

Anyone who is sexually active takes risks.  If you're in this game, you'll spend time with girls who are sexually hyper-active.  Many partners (even if they claim "low volume") and those partners may in turn have many partners.  Think six degrees of separation, but with a lot of dicks and twats.  That is a risk.  Proper use of a condom every time helps reduce that risk.  But condoms fail sometimes.  Also, condoms don't prevent transmission of all STDs.  Some STDs can be cured with a simple treatment of antibiotics.  Some will be with you for life and are life-threatening or nearly so.  Think about what level of risk you personally are willing to accept for this.  Based on your question, this game may not be one you really want to play.

I truly appreciate it.  I've tried to research the STD topic thoroughly to gauge the amount of risk.  There seems to be wildly different opinions on the amount of risk involved, from "you're pretty safe if you stick with TER girls" to "expect to catch something anytime you've seen a provider".  I'm sure the truth is somewhere in between.  I'd like to think if a provider knew they had something, they would take steps to ensure they weren't spreading it to others, but I'm not naive enough to think that is universal.  Like anything, I'm sure some do and some don't.

Back channel as a relationship with people I know from the boards. I doubt I'd even answer a PM from a stranger asking about "STD" issues. That just sounds a little odd to me

Unfortunately, there is not much data available on the prevalence of STDs like HIV for those ladies participating in the oldest profession in the U.S. and Canada.  The CDC website basically throws its hands up on the lack of data, suggesting that the stigma and legal prohibition in many countries is to blame.  Limited data available in Europe has the rate of HIV there for working girls at around 1-2% on average, but with higher rates in Portugal and Spain (~13%).  For reference, the overall prevalence of HIV in Western & Central Europe and North America is estimated at 0.2%.  But the European data for working girls can be difficult to decipher because some of it lumps upscale providers in with SWs.  With SWs, there are other factors present that increase their risk much more than the number of partners: inconsistent condom use and - probably worse - IV drug use (and abuse).  Upscale providers tend to use condoms properly and consistently, most aren't IV drug abusers, and many get tested regularly.  But that can vary from provider to provider.  Unfortunately, due to the prohibition approach taken by the U.S., the industry is not regulated as it is in some other countries.  Hope this helps for perspective.  Good luck!

...picking up a civilian at the bar and going home with them. Most people over-report condom usage when asked about it due to morality issues, and wouldn't actually discuss lack of condom use if asked.  

Since providers literally have sex for a living, and their businesses are based on the assumption that they are in good health, the chances of actually getting something from a provider are a lot lower.  There are a plethora of other reasons as well, but i don't think this is the place to get into those issues because they involve demographics and socioeconomic theories that surround what populations are most likely to have STIs.  

Hookers might be really great at dirty talk, do dirty things in bed, and get dirty if they fall into a mud puddle, but by and large, hookers are the largest proponents of safer sex out there.  Couple that with being tested on a regular basis (which you should be too!), and the risks are reduced significantly.  

PS:
Lots of STIs are asymptomatic, and can be for life. Hence why 60-80% of the population has one type of HPV or another and HSV. Also note, that just because you are exposed to an STI does not mean you will seroconvert. STIs need specific environments in which to proliferate, and those environments are specific and tenuous at best. This doesn't mean one should be haphazard about safer sex practices, but one also need not be whorephobic when it comes to getting an STI (also take into consideration that the penetrated party is far more likely to receive and STI than they are to give one). "Expecting to catch something anytime you've seen a provider" is seriously sex-negative, whorephobic, and assumes that hookers have no scruples, morals, or awareness when it comes to their health and the health of their clients.  

You are just as responsible for the health of a hooker as she is for yours, and you're even more responsible for your own health than she is.  So be responsible about your own health and stop dumping your fears onto a hooker you haven't even stuck it into.