The tip is not mandatory, but definitely expected if you want a valet to take excellent care of your vehicle. The tip tends to be expected on arrival and departure, as you may not have the same valet both times. This can be prove to be an expensive service when you add tips for valets.
Handing over your keys over to the valet gives him full control of your vehicle. An article on BuffaloNews.com reported of a valet impostor who tried to steal a Mercedes-Benz. Making sure that the valet shows an identification card is wearing the correct uniform usually reduces the risk.
No matter how good the valet is, there's always the probability that she might scratch or dent your vehicle. Some of the problems that arise are accepting responsibility, notifying the owner of the damage and maintaining the proper insurance.