I read that tipping when you pick up your car is considered reasonable and customary, so that's what I do. I know that a different valet from the one that parked my car will likely fetch it, but I'm sure that it all averages out over the course of a shift. I usually tip $3.
I valet parked at the Contemporary for a Chef Mickey's dinner on our last trip. The valet mentioned to me that her shift was ending, and that someone else would be retrieveing the car for me. I said OK. So she repeated it. After she brought it a couple more times, the light bulb finally went on and I gave her $2. It's the only time I've been pressured to tip at WDW.