Yeah, that's what I do too.
And as far as I can tell, usually when they provide generous "extras" without prompting, it's because they are personally into it, so I guess that could be considered more a fringe benefit of the job Guess it depends what shape you keep yourself in, etc. I work out a lot, which is really why I get massages in the first place. So should I tip less because when I work out, I might be more pleasing to rub down than if I wasn't busting my butt at the gym lol? I'm kind of only half kidding, since personally I really enjoy getting my hands on another in-shape body.
But I also have worked for tips before, and I realize it's strenuous work and rent is high. So I would tend to tip more to guys who appear to pay a lot to have nicer places vs guys living in dumpy one-bedrooms with a roommate. And I also would tip more when guys seem to under-price themselves for whatever reason. So if a guy is asking $90 and gives a great massage, I might tip $40 or $60 whereas if a guy starts at $150 for the same or lesser massage, seems to make sense to tip less.
Just my thoughts, anyhow.