I suspect, although I do not know for a fact, that if we magically knew the STD status of every Montreal provider the fewest STDs would exist amongst providers at the best known and most expensive agencies (and amongst the independents charging relatively higher hourly rates).
Patron, while your admittedly speculative conclusions seem intuitively reasonable (and perhaps Aristotle would agree if he was moderating this discussion), the degrees by which the risk is lower is so negligible that I would contend that they are not even appreciable, and do not matter, once you have made the decision to accept some level of risk.
It's a completely fucked up mindset to think that because you booked a Euphoria girl, or some other "approved" agency girl, that you don't need to worry about STD. It's an equally fucked up mindset to believe that a sexual experience with a civilian lady you meet at a bar is "safer" than seeing a Euphoria girl or approved agency girl or approved indy. People who actually in reality think this way should not even be on this board.
What's rarely discussed on this board in these STD threads are sexual practices that seriously expose one to STD. Apart from BBFS, duo sex is very risky if a male first observes female to female cunnilingus and then receives fellatio from one or both females without first compelling them to receive a vigorous rinse of listerine in their mouths. There are people out there who ask the same questions as the OP and engage in these risky practices on the belief that they made a "safe initial choice", thus somehow, some way eliminating the risk of the subsequent behaviors. One need not be a learned pupil of Aristotle to see the complete flaw in this logic. The STD threads I have read on MERB since its inception invaiably contain this kind of "self fulfilling logic" which is more designed to justify desired behaviors than to reflect the scientific reality of the safety of those behaviors.
In the end, we all have our own opinions and very little or no data on these kinds of issues, and we make our own choices with the comfort level we each have with the perceived risk.