A valet service (where your car is taken at a designated area like the front door of an establishment) can either be a charged or non-charged service but that should never stop you from tipping the individual who whisks your car away and retrieves it when you return.
Be friendly and courteous with the attendant. In most situations these individuals come across many high-end vehicles throughout their shifts and are not eye-balling your car (be it Ferrari or Ford) for any intended joyrides. Presuming that every attendant that comes into contact with your precious ride is a scheming hoodlum will not win you any awards and it will definitely not win you any extra care or consideration for your car.
An average tip for valet service is anywhere between two to five dollars depending on your generosity and should be handed to the valet in a courteous manner (tossing of coins in their palm or general direction is just not nice!) when you are picking up your vehicle.