It's true that traditionally a tip was only expected when the valet returns you car to you, but, needless to say, a tip is also appreciated when you leave your car with the valet.
15 years ago, I would tip $2 only when the car was returned to me. Since then, the cost of everything has gone up — and my salary has gone up too. Now, I usually tip $2 both ways. And I gladly tip more for especially attentive, quick and friendly service.