Unless she's an agency provider or you're at a massage parlor or peep show or some other non-indy arrangement in which the company she works with/for takes a large portion of the rate, tipping is NOT customary. Whether it's
deserved or not is another argument.
Did she go above and beyond your expectations? Did she go above and beyond your expectations -- especially considering the rate she's at? Is your wallet just feeling extra heavy that day? Just overwhelmed with endorphins and want to spread the joy? Go ahead and tip. Otherwise, she'll set her rate at however much she expects to be given for the specified time you spend together, and that's what you should give her.
Now, gifts, on the other hand, are something I DESERVE, and I have a note from Santa to prove it.