There are too many factors involved in saying how much.  I'll say this, if you see 6-7 providers a year, your chances of being **exposed** to (not getting) an STD are close to 100 percent.  

That having been said, this doesn't mean that you will catch one.  It's not widely reported, but your body defenses will take care of most of them before they get a foothold.  And most STD's are aggravations, even if you don't want to pass them on.  I like to say if you were playing tennis instead of having sex, it would be like having a strained knee.  Except for AIDS or Herpes, or other viruses like warts, they are generally not catastrophic if they are treated.  But don't trifle with them, either.    

You could take other precautions to reduce your chances afterward.  A shower with a thorough cleaning of your genitals and anus, (if applicable) and a cleaning of your mouth including antibacterial mouthwash will reduce chances of catching them.  They have to gain a foothold.  Also the use of Neo- or poly-sporin on your genitals afterward will prevent a lot of them.  However, you're not likely to be so active after 1-3 orgasms.

To your questions:

1) Asking her if she has any STD's will be ineffective, as most STD's can be asymptomatic.  Also assume that with her pay on the line, she probably won't be inclined to be honest just then.  It's better to inquire if she has been tested, and the time of her last test.    

2) No, I wouldn't presume that a provider would give a straight answer.  Her clients would bail.  It would be a disaster for her.  The most I hope for is that she doesn't have an active attack, (it would be very difficult to go through a session with that) and that she's on an antiviral like Acyclivir or Valtrex that lower the chance of transmission.  For oral herpes, BTW, 80 percent of the population has been exposed to it.  They usually never have symptoms.    

3) In avoiding "uncovered" activities, of course you are less likely to catch STD's.  However, the grand-daddy of STD's, AIDS, is almost never transmitted through oral sex (I think there's been one suspected case), though.  You would have to include kissing in that too.

/Zin

I agree with the previous sentiments. I assume that anyone I am with outside of a totally monogamous relationship, has a possible STD. Even the easily treated STD's can cause incredible turmoil for a guy that passes it on to his wife or SO if she is not aware of his hobby.

Even if the provider you are with is tested regularly, she could still get infected by the last guy she saw. I work with many college age students in a health care setting and we have seen a few gonorrheal throat infections likely from performing receptive oral sex. Gonorrhea can live in the throat and be spread to a penis from there by BBBJ. Some researchers believe that chlamydia can also infect the throat. The provider would very likely not even know she has this infection as it does not always cause a sore throat. I am much more comfortable with CBJ for this reason.

Herpes Simplex 2 (the type of herpes that makes its' home in the genital region) does not do well living in HSV1 territory (the oral area.) HSV 1 can live in the genital area but this infection tends not to be very severe and tends not to recur. It is also easy to explain as nearly everyone has HSV 1 orally. I suspect that masturbating with your own saliva could possibly spread it to the genitals (yea, that's the ticket ...)

Herpes Simplex 2 can be spead from genital to genital contact. Condoms do not eliminate this risk. HSV 2 can live anywhere in the area generally covered by jockey shorts. If you do FS or even privates to privates massage, you are at some risk for getting HSV 2.

As for DATY, I really am hard pressed to find any real danger with this, provided she is not bleeding at the time. Maybe hepatitus A or B but I really think that is a long shot. Could you get a gonorrheal infection in your throat from DATY if she is infected and pass that on to your SO by DATY? Seems like a long shot to me.

Here are some links that may help:
http://www.herpes.com/hsv1-2.html
http://www.dph.sf.ca.us/sfcityclinic/drk/womenshealth4.asp
http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Hepatitis_B_the_facts

I doubt most providers are frquently tested for STDs.  If you ask a provider about STDs I doubt you will get a truthful answer.

If that is of importance then I suggest hobbying in Nevada's legal brothels.  Or in Tijuana.  I've never asked a girl in Nevada to show me her health card.  I have in Tijuana. (To avoid arrest in TJ a chica must have it in her possession.  Believe it or not, many do carry it with them and if you insist they will show it to you.  I'm speaking from experience:  been there, done that.)