We are offering something very personal and hopefully we are connecting with our clients in a different way than a Dermatologist (who, in my experience, have been impersonal), accountant, bookkeeper, etc. It's not about the money that everyone is talking about but a way that clients can say, "thank you" if they choose to. I do not agree that it demeans us at all. When I get services like this done, I WANT to tip. And, if they are good, I'm going to also refer them to my friends. I don't need to be told to do it.
I go out of my way to make each of my clients feel special. I always give more than they expect. My accountant is not going to do that and I would not expect her to either. If my clients want to tip, I appreciate it. If they don't, I still appreciate their business. I never remember who tips and who doesn't.
That being said, it's your own business and if that is what you want to do, go for it. One of the great things about renting is you get to make all the decisions.