If you want your car to receive good treatment, I recommend tipping on arrival.
If you tip only when you leave, how does that influence the care your car received?
It doesn't, unless you are a regular customer of that particular valet operation.
If you tip generously on the way out and you are not a regular valet customer at that location, and don't expect to be a regular valet customer in the future, it doesn't do your car much good. Because they will treat your car like everybody else's.
But if you tip upfront, when you arrive, especially if it is a generous tip, this influences the care of the car. Unless they are a rookie. In this case, a rookie will not appreciate your gesture like an experienced valet will.
Experienced valets are more likely to give you value for your generous upfront tip by parking your car in a better, safer space that has more room, is exposed to fewer cars, and is less likely to be bumped, scratched, dented, dinged or scraped.
Experienced valets who receive a nice upfront tip are more likely to show your car more respect, to drive it not quite as rough, and to suppress urges to do monkey business for the next car.
What's a good upfront tip? I think $5 is good. Ten is lavish. Twenty is incredibly generous.
So should you tip when you get the car back?
As a courtesy. The valet team may have had a shift change. I think a minimum tip of $2 is sufficient on departure.
So deploy the bulk of the tip money on arrival to influence the level of care shown to your car, and to you. And tip a smaller amount on departure, as a courtesy.
Just make sure they did a perfect, flawless job when you get your car back. Look it over thoroughly. I mean crazy thoroughly! If something is amiss and you find it later, you're screwed.
Find any damage now, before you drive off the valet ramp.
Hopefully, your generous upfront "preemptive tip" will have increased your chances of averting what can go wrong when using valet parking services.
It incentivizes them to care. It rattles them out of the depressive state that happens when they are working a crappy shift yet again.
It communicates appreciation for their work.
It tells them this person knows how to tip. Good tippers usually get the best service.
If you tip nothing, they hate you. Try not to park with them again.
If you tip a dollar, they really don't like you. Same if you tip in coins. They don't want to run to the garage with a pocket full of change that can spill-out as they move in and out of drivers' seats.
Upfront tipping is the way to go, in paper currency.