Dean is correct that most published guidelines for valet parking tips indicate $2, and only when they bring your car to you (not when you're dropping your car off).
For example, see
http://money.cnn.com/pf/features/lists/tipping/
However, that's the same amount as 20 years ago. People in tipped positions who earn a percentage are earning twice as much as 20 years ago because of inflation. But those who get an unchanging set amount, such as valet parking attendants, are earning less and less in real, inflation-adjusted dollars.
So I'm happy to tip more than the "standard" $2 for friendly, attentive service. Although I agree that a tip when you're dropping off your car is optional, I don't mind tipping a valet parking attendant who deserves it at drop-off or pick-up.