Usually valet parking is never completely free.
To clear up some confusion... unless it is made clear that tipping valets at a certain location is not allowed, then you should be handing out gratuities.
If you are attending a wedding or other event and "hosted valet parking" is offered, this means your host is paying. It's free for you. But unless an invitation or party announcement specifically says the valets' tips are also taken care of by the host, then YOU should be the one to tip.
If there is hosted parking at a luxury property and you are assuming the valets probably are well paid and tipping is probably an unnecessary extravagance, you are wrong. In nearly all cases valet workers are tipped employees. Wages are as low as $2.13 an hour in 19 states.
Tipping is expected.
Typically, valet tips are pooled. Every time a valet worker arrives or leaves work, a "cut" is done where all the tip money collected by the team is equally divided among all. (One dollar tips don't help much.)
With consumers, the longstanding custom is to tip only at the end of the valet parking transaction, but this is a flawed approach.
I recommend you tip on arrival and departure with the bulk of the tip being deployed on arrival in order to influence the quality of care your vehicle is about to receive.
Tipping only at the end does nothing to influence care. I wrote a lot about this in: "What to do when valet parking."
Additional useful article: "How much to tip valets."
If too many guests stiff the valets, it can be embarrassing to the event host.