Yes I agree, I doubt they would even talk about it amongst themselves (despite some claims here) because the one case that I did hear about turned out to be untrue and was started by another sp due to professional jealousy I guess. The number of clients that limit their activities to oral is probably small except for maybe massage johns. Have also heard very little from Johns on the board who have contracted std's, if anything.
My own personal position is that the risk is very low with high end girls who take care of themselves, but everyone should make their own choice and get tested periodically.
IME, dollar for dollar, more attractive female SP's tend to perform at a much lower level of service than their inferiors. In fact, that may also be the case when they are paid significantly more. This includes an unwillingness to get down and dirty, which is related to the STI risk. Perhaps this applies also to the "high end" girls you speak of.
Moreover, if such girls have fewer customers than other SP's, all else being equal, then they would be at less risk.
Certainly i'd expect them to be at lower risk for STIs than IDU drug addicted SP's. I've also read reports that SP's in Australia have a low incidence of HIV and that certain Canadian escorts studied had a low rate of STDs when compared to the general population.