In classes - - I've only been tipped 3 or 4 times out of a hundred students or so [It was again in the 15% region or so - VERY respectable when compared to what I'm paid!] - - In general -I try to make sure that goes to the DM since they aren't paid at all ~!
200 or so students, 3 tips as an Instructor over the years here.
As far as DMs not being paid, that's definitely a situational/regional thing. Most operators I've seen here probably pay the DMs as well or better than the Instructors.... it works sort of backwards here, but Instructors can be a dime a dozen as good numbers of newly minted Instructors make their way to the islands, and ops will hire them, work them with students and intros 'til they know the sites, then they've got a shot at straight DM work - if there is a pay differential, it goes to the people who've been around longer, which happens to be the DM over the Instructor in many cases. We've had a few operators here that can somehow get people to work for free, but that's not the norm.