First off, we feel the need to emphasize that even though free valet parking is being curtailed up and down the Strip, it shouldn't affect the size of the tip to the parkers.
Of course, that's not to say that it won't, human nature being what it is. But we feel it shouldn't.
Next, as to the question of when to tip the valet(s), the consensus online, which tallies with our personal experience, is that you tip the valet who retrieves your car. Although the job is done, so the toke has no effect on the service, tipping on delivery is standard guest procedure (no different than tipping a waitperson when paying the bill).
On the other hand, there's also a school of thought that holds you should tip the valet when you're dropping off your car. After all, how often will you get the same valet when arriving and leaving? By only tipping the guy who returns it, in effect you're tipping for only half the service you've received. The reason we don't agree, under normal circumstances, is 1) we believe you're paying for the round-trip service; and 2) all valet departments we know pool their tips, so it's not like you're stiffing one parker while tipping the other.
On the other other hand, if you want special service up front -- for example, if you'd like your car parked very close for a quick getaway or if you want to tip the scales in favor of a little extra TLC while parking your car -- tipping when you drop off your car can be a good idea.
In addition, when you pull up to a valet parking area and see a sign that announces, "Valet Full," with a $10 toke up front, an attendant can often miraculously find that one last space for your trusty steed.
And if you hand over the lucre with your valet ticket when you pick up, you can often jump to the front of the line (if there is a line, which there will certainly be when coming out of a big event at a big casino).
Definitely make sure there aren't any valuables visible when you valet park. The vast majority of attendants are scrupulous, but it only takes one who isn't for something you value to wind up missing.
Also, if you can, take a quick look around the car when it's delivered back to you to check its condition. If there's damage, that's the time to try to do something about it. But don't be surprised if your valet quickly disappears and no one stops to help you ever again.
The recommended tipping amount varies widely. A dollar tip marks you as a stiff, sometimes even more than not tipping at all. (It's like leaving your waitperson a penny or a quarter.) A $2 toke is just slightly better, while $3 puts you in typical territory and $5 is a great tip. Ten dollars is lavish; $20 makes the valet department's day.
According one valet parker, most locals tip $2-$3, while visitors go the full $5 ("because that's what the travel blogs tell them to do").
Finally, what do valet parkers earn? According to Salary.com, a parking attendant can expect to make a base salary of $18,000 to $29,000 a year (depending on seniority and the establishment's policy). Add in tips and that number can go as high as $70,000. One valet told the Review-Journal that he averaged $150 a night in tips –- and on the graveyard shift, to boot. "It’s not uncommon for me to get tips in the hundreds range. This is Las Vegas. We’re a city of high rollers," another valet told the R-J.