I've been tipped many times, given lovely gifts, and was always delighted, surprised, and grateful for such generosity. All I ever anticipate is the stated rate for the time spent in my company.
However, I don't think what a person makes or his profession has anything to do with whether or not she/he deserves a wee bit extra after a job completed diligently. I "tipped" a contractor for finishing repairs at my house ahead of schedule, touching up chipped paint that had nothing to do with what he had worked on, and cleaning up debris left behind PLUS lugging off some heavy stuff I had sitting that needed to be trashed. Bonus, tip, whatever, he did it without asking and I gave it by my own choice for out of the ordinary courtesy beyond the normal parameters of the task.
What exceeds expectations is in the eye of the beholding purchaser. Best to strive for exemplary service out of personal pride, if nothing else, and the desire to leave a client happy and satisfied.
By the way, I have a couple of kissable, pink tips at the pinnacles o' the twin peaks o' Ireland ready for appreciation of a project well done.