My attitude is if you own your own business, you don't get tipped. It's like hairdressers who work out of their house. So, if I know you're working for a pimp (but then I wouldn't see you) or agency, I'll tip. If you are independent, then no, since the price you set is exactly what you're getting,
Having said that, I have tipped regulars during the holiday season.
But I haven't seen anyone in so long, I can't remember how to pay, never mind tip...