This is a thorny question. There is an overwhelming consensus online, which tallies with our personal experience, that you tip the valet who retrieves your car. Tipping upon drop-off of your car is totally optional and is not done by the majority of drivers. "Tip your valet when your car is returned to you from the parking lot," advises SmartWomenDrivers, which goes on to advise that you should definitely add a gratuity if your car is being parked in inclement weather. Help with luggage also merits recognition. Although he's already done the job and, therefore, the tip can have no effect on the service he provides, tipping upon delivery is standard guest procedure.
However, there is also a school of thought which holds that 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 have received. Of course, if you receive poor service, not tipping sends a message, too.
Also, tipping when you arrive incentivizes the valet to give your car a little extra TLC, such as finding a prime parking spot for it should you indicate that you'll be in a hurry to leave. According to ArtOfManliness.com, "Take care of the person you are entrusting with what is likely your most valuable possession (besides your home). Even if it's not intentional, as humans we tend to take better care of items in our care if we've been treated well and appreciated." It also suggests that your valet might be able to find extra space in a "full" garage if you negotiate the tip upwards from, say, $5 to $20. A thoughtful bottle of water from the valet while you're waiting in the Vegas heat also merits extra recognition.
"Start by putting a tip on the dash. What that does is it helps to influence the level of care your car's about to receive. If you only tip at the end, there's no way you can undo the care or lack of care," parking valet Ed Ryder told ABC News. (He also recommended taking all your valuables out of the car and – somewhat impractically – going over a checklist of the car's condition when you drop it off.) It's also advised that you inform the valet if you're going to need the car later that night. "In the past at our property, I know that joy rides were happening," said Ryder, rather alarmingly.
The recommended tipping amount varies widely. Some suggest that it be at a ratio of one dollar per every $10,000 of the value of your car. That would not work out well for the valet if you're driving an old clunker. Two dollars or less marks you as a "stiff." Five dollars is considered generous and if you're feeling like a "whale" (and we don't mean from gorging yourself at the buffet), you cross the valet's palm with $20 or so. However, since paid parking is coming to the Las Vegas Strip, we expect extravagant tips to suffer accordingly (see that 07/08/16 QoD referenced in today's question).
