@sometinfishy: You're spot on with the observation here probably most of the time.  Who cannot use a little extra money?  It can be used for something practical: pay off some college loans, insurance, or car.  Or it can be used for savings or for something fun like a nice dinner out, a concert, drinks, or a game of golf (do any girls here play golf?).
@BB: Definitely a return visit is a great 'tip' and is a sign of satisfaction.  For me, I think that it would be a little awkward to pull out my calendar and check out times then and there.  For dating, don't we say, "I will call you ___ "?  Then either we do or we don't.  (Myself, I won't say that if I don't mean it.)  And then we call later and ask about the possibililty of another date.  Afterall, maybe the provider doesn't want to meet with the client again. 
@Allie_Kat: I myself would prefer pizza and beer.  But then, the gift we are discussing is given by me, not given to me.
@DallasRain: That looks pretty cool (although I am not sure exactly what it is).  Sure it might be easy for some people to see a tip/gift as a measure of performance, and you and I both disagree with that.  To me, it is a small measure of appreciation and gratitude.
@catnipdipper: I agree that it is always best manners to be aware of not overstepping your time.  If you are having a good time with an attractive woman, that can take some serious discipline.  Don't guys in real life often get accused of a quick climax and then rolling over and going to sleep when the girl wants some more touch and talk and maybe some Os?  Well, maybe a provider/client date is, at least, a little different.
@bustymaryjane: It seems that the right thing to do is to give a gift/tip not in order that I will benefit even more but in a spirit of gratitude (of course that doesn't mean that one is not aware that one's gift might likely be countered with a corresponding measure of gratitude 

)