I only use valet service when forced to by hotel parking policies. My understanding has always been that you tip only at pickup and $2 - $3 is pretty typical.
DS20 got a summer job at a nearby Hilton as a valet and the tips are all over the place. Many people tip nothing at either end. Many tip very generously. He also (surprisingly) gets tips sometimes when he doesn't even park or deliver a car. When he works the night shift, there is no bellman on duty in the wee hours of the night, so people ask him things. He knows the area pretty well and has learned the answers to the typical questions that he didn't know. He has enjoyed the job.
Sheila