To clarify, I was not suggesting that I would have unprotected sex with an escort based on test results or that anyone else should do so. The fact that testing can be anonymous so that an escort could pay someone else for his results or pay someone to take the test under the escort's name, etc. ensure that I will never trust or rely on such results. On top of that, even if the results could not be faked, the results cannot show whether someone was infected with certain STDs (including HIV) within ~90 days before testing. So I would never agree to have unprotected sex with an escort.
Any time I see an ad that says the escort does not practice safer sex all the time (e.g., the "sometimes safe" answer on Rentboy), I move on to the next guy. I know people lie and I've seen escorts change those statements from sometimes safe to always safe, but I definitely want to skip the guys who admit to having unsafe sex.
In a story similar to the one shared by Lohengrin, I was with a popular escort in NYC a few years ago and it was my third time hiring him. The first time I hired him, I thought he was trying to fuck me without a condom, and I stopped him. It turned out that he had a condom on, and I felt slightly embarrassed. I was glad I stopped him to check, and I definitely was not so embarrassed that I regretted stopping him to be sure. Fast forward a couple of visits, and I did not check. During the first visit he said he always uses protection; so I didn't think I needed to check him. Big mistake. After a couple of minutes, I realized he was not using a condom. Needless to say, I was furious. I was even madder when he started laughing and said I should be honored that I had earned his trust. I informed him that he had forever lost my trust and told him to get out of my hotel room.