I have done it both ways. Do what feels comfortable for you.

you feel like doing so...or because you think it is appropriate?

If you search on this board for this topic you will find many discussions on this which may assist you in your answer.

I have never tipped a gal, nor will I ever tip a provider.  My rationale is that they are not working at less than minimum wage (not even close) making a living (as a waitperson might) on tips.  Others incorporate that into the session fees, and they have their own rationale why they wish to tip.

Do some research here and make sure you understand the differences in why/why not tipping happens...or doesn't.

I had one instance crop up...  a certain private "Massage lady" set her rates for massage (FBSM w/ HE).  After a few meetings, she let me know that "extras" were available.  No discussion of a added donation...  the activities transpired.  Getting dressed, she refused to even suggest a donation even when I pressed.  Talk about arkward.  I definately felt pressured...  by myself.  It was pretty much "tip what you think is right".  She said she'd be happy with what ever I gave.  I think this is a tactic so LE can not set her up.
I get as much time as I need...  plus FS...  In the end I decided to compensate her the same as I did other persons who advertize as an escort.   Now I continue to see her, I set the rate myself...  and that is how much I leave every time.  
She is UTR & not TER reviewed though I found her through a now defunct web site...
skb