I agree with Eva and Brooke and I certainly see their point of view, and I understand the OP's concerns. It is the hobby after all and aside from catching anything nasty orally, you can catch things just rubbing the genital area of a person regardless of having a condom on or not. Herpes is easily transmittable even when there is no current visible outbreak. Google it and see, it has to do with the shedding of the infected skin cells.
However, with that said I have seen various boyfriends over the past 10 years and "knock on wood" have yet to catch anything. But I am very YMMV and I take the time to do certain things that every lady should do which includes but not limited to a visible check as well as some other methods. (pm me ladies if you would like more info on how to do this from an experienced lady).
For the newer gals they really should look up the SWOP
http://www.swopusa.org/ site or other sites like Coyote
http://www.bayswan.org/coyote.html and such that give directions on how to check a guy and be safe. This is pretty standard as well at the famous Las Vegas Brothels. Guys if you ever decide to go to one of these in Las Vegas prepare to get a light shined on you and a visible check done prior to any activity. It is done very clinical like. Some of the brothels make it mandatory to use a condom and dental dam for any oral.
If you see something that doesn't look right or smell right, you either pass on giving oral all together, or slap a cover on it, depending on the circumstances.
The ladies who last in this little hobby of ours get regular STD testing done, (you must invest in this), and if you are a guy who hobby's regularly with the ladies you need to do the same.
As I have said before in other threads, guys if you have unusual bumps or things that are on your genitals don't assume it is benign or that its a skin tag or something harmless. Go to the doctor get it checked and treated. I mean after all your spending anywhere from 200 on up to visit ladies, so you can take some of that money and invest it in regular checks and treatment if necessary at a doctors office.
Common sense should be paramount. I know, I know, that's asking a lot from some people.