Just because you tip doesn't necessarily mean that the service or mileage will get better ... I'm sure MPA's appreciate it, but why spoil them to begin with. If I get the highest option in a session, I'm sure the MPA is happy with making the most $$$ for the time spent with me. I do tip on the rare ocassion, or those MPA's who constantly go out of their way for me, but apart from that, if I get average service, I don't bother, as I know a repeat is not on the horizon anytime soon.
Many MPA's are quite satisfied if you treat them with respect, and repeat with them if you were happy with the previous session. Just speaking from my own experiences.