I'm not averse to tipping, btw.. but I do it based on overall performance rather than mileage. An intense , passionate session with a girl who's really into it, but with no extras is much better than a disinterested girl who provides a lacklustre and mechanical session and BJ and dead-fish FS.. and I prefer to reward accordingly.
Plus I hate talking about money in the room. Once things get started, I want the GFE fantasy.. and nothing snaps me back to reality faster than talking about money.