I guess I always thought the rule of thumb was tip for a service. Tip the barber, the luggage carrier, the car parking dude, the waiter, waitress, bartender, the guide and so on. I also thought a rule of thumb was that you do not tip te owner of the establishment. The barber who owns the shop and so on.
By that rule of thumb you should tip the dentist. I promise you that isn't going to happen from me!