You should practice in what ever way makes you comfortable.

If you do everything covered, you should be all set, but if the fear of STDs makes you uncomfortable then this hobby is not for you.

You can do some research on STDs by going to The Erotic Highway Board and doing a search on there for the CDC.  You will then be led to sites that have a wealth of info on STD occurances associated with various sexual practices, etc.

And for my part, I have been hobbying for over 35 years and have not once gotten an STD despite using condoms only for intercourse.

The purpose of the hobby is to bring happiness.  If it does not do that for you, it will be a waste of money and then some.

Best luck with whatever you choose.

If a provider knows she has an STD and is working despite that, do you really think she's going to tell you?  Thus the question is pointless and mrfisher's advice becomes the only issue.  Even if she says she's been tested, she (and you) can't really know for sure.  The good news is, a high end provider is most likely to be 1) low volume and 2) careful.  Most people will tell you there's a better chance of getting an STD from a civvie you pick up in a bar.  Wishful thinking?  Perhaps.  So if your concerns seem more than you can deal with, simply avoid DATY, DATO, DFK/LFK and BBFS.  It takes away a lot of the fun, but at least you'll be safe.

would you do if she said she was contageous?  This hobby is about having class and treating ladies with respect.

Stay home.  That is the only way to guarantee zero risk of catching an STD from a provider.  

Asking her when she got tested is a waste of time.  She could have a raging case of just about anything but if she has decided to keep working she clearly doesn't care who she gives it to and, of course, she is not going to be honest about it.

There is NO sex, paid or unpaid, without some risk.