A Little? Let me tell you from experience, we can size up the john in a snap, know what he wants, (or needs) and choose a line to use to reel him in. You are correct that hustlers do the least for the most, we also know the john MUST cum, and the better we do to achieve his orgasm, the more likely we are to get paid well, and maybe even stay for a night. (If the room is his, in the first place). Lots of times I would forego my orgasm because I knew I was gonna be back on 7th st. in just a few minutes, but, occasionally, a guy would happen along, who wanted more than just sex, and of course I played along, even though we both knew I was still leaving at some point. I had a guy one time tell me flat out he wanted to save me from the streets, I didn't have the heart to tell him I lived in a seminary!
I notice I used the present tense at first, but I assure you, I haven't been hustling for many many years now.