I've punted several times in both the US and in Germany. In the US, tipping a WG is not the norm but definitely more commonplace than here. It is generally suggested, although by no means a given, that if you have or want a regular girl, tipping "shows your appreciation". Thankfully I don't care for a regular US girl so I have never considered it.
In Germany, you are a mug if you tip. It's just not the done thing. You have paid for a service and that is that. The way you get favourable treatment is to repeat regularly. That way the locals get the favourable treatment over the tourists.
That's the approach I take. I have never once thought about tipping a girl, no matter how amazing the sex is (and it has been mind-blowing a good few times). Tips should be in the form of good reviews and repeat business. Nothing more.
