So in DC where there ain't no lappers allowed, and being the Prince of PL's, I will indeed tip a dancer who sits with me and gives me "access" while we're hanging out. As a general rule I do buy drinks and sometimes will pay for food (that's a dayshift thing) in addition to posting up some cash for the pleasure of their company.
The way I frame the tip is by saying something like..."Hey Summer here's a little tip for spending time with me this afternoon. I really enjoyed the conversation and it was fun hanging out with you. Perhaps we can do it again the next time I stop by." Which is usually followed up by a hug and a little kiss in appreciation of my generosity.
My rule of thumb is $20 an hour or part there of.
Before you flame me for my for my extreme PLness this is just in DC where the club scenario is a bit fucked up and the behavior I describe is by and large the norm. Additionally if done correctly it can (and has) resulted in OTC adventures from time to time.
Out in then real world my sentiments are pretty much in alignment with the I'll tip them by paying for dances and other "favors" crowd with some exceptions.