I'm generally against any form of tipping, be it at restaurants and certainly not with WG's. On the odd occasion, I may leave some loose coins behind after a meal but its not my job to top up a waiter/waitresses salary because its the expected thing to do. There is a minimum wage!
Tipping a WG is a cue for her to raise her prices. If £100 an hour isn't too steep for some, that they want to tip an extra tenner for the purposes of looking flash or hoping it will get some other benefits, its foolish. If a WG is generally charging less than the market rate and I have a very good session then I may be inclined to add a little something but its very rare.
If I want to tip someone or have extra cash to burn, plenty of charities out there who are also very deserving of an anonymous tip.