Thats an interesting thought. I understand the idea behind tipping in the U.S but here in Australia and Asia we don't tip DM's or Instructors. It's never been an issue and I guess in a way that makes service easier to deal with.
We all know that you don't become an instructor for the big bucks, you do it for the love of the sport, and because you want to introduce people to the right way of diving. It all comes under the same guise. DM's generally don't get paid here either, but there are other perks (free courses, free gear, free trips) that more than make up for it.
DM's do an amazing job and because instructors were DM's at one stage we know what they go through, the work and effort that they put in and whats behind their motivation.
If you get tipped, good on you, but we don't expect it and give the same service with that in mind.
Coogeeman