My thought, as with all services, is that if the service provider (waiter, barber, instructor) is running the business, they should charge what they need to make and no tip should be involved. If the service provider is working for another (restaurateur, salon, shop owner) who is taking most of the profit (which can be hard to determine, but I usually assume they are) and paying the actual instructor a pittance, tipping is appropriate, appreciated, and in some cases expected.