i'm not sure what ms. manners would say, but i for one completely love the valet service and always tip $2 at drop off and $2 at pickup. Its worth it to me and the valet cm's are always so nice and helpful. Haven't had a bad experience with them yet.
We usually tip both pick up and drop off at WDW because it is usually different guys. If you are just going for dinner, it is okay for only tipping on picking up. Tips are your discretion. When we stay at a resort and use valet during our entire stay (highly recommended!), they get to know us and we get awesome service (unloading our groceries etc)!
We usually tip both ways -- $2 -- but often, we end up dropping off at the valet stand to the "manager" not to a valet. In that case, we don't tip dropping off. But ALWAYS $2 picking up -- more if it's raining, extra hot, extra busy, etc.