Understandably we haven't heard from any of our more user friendly WGs, but my feelings are, don't tip a lowly paid waiter/waitress in a restaurant and they would be wondering how they have offended you, tip a very highly paid WG and she certainly wouldn't be expecting it, and indeed might be highly suspicious as to your ultimate intentions. To my mind tipping might come over come over as being somewhat creepy!
Sometimes I might give them a little something if I have fucked up for some reason and they have accommodated me (say a bunch of flowers if I was fifteen minutes late for a previous meet and they have given me my full time), but only for a specific incident of them going over and beyond normal good service, which, as I said, is rewarded by repeat bookings.