Here's a little background, spoken from the perspective of a valet parker:
I work for a major hotel/casino as a valet parker in an area where parking spaces are a premium. Valet parking is provided as a free service by the business, and is not contracted out.
Self-parking is provided at a fee, but is refunded for guests who actually utilize our business, rather than go somewhere else (almost like validated parking).
The environment can reach upwards of 110 degrees regularly (summer), and a low of 30 degrees (winter). Vehicles range from clunkers to luxury vehicles. Cleanliness of vehicles range from garbage bins to professionally detailed.
To begin with, the above article is pretty close to accurate, but some things should be clearer.
It is generally good to tip a valet when they take your vehicle, and to also tip when the vehicle is retrieved ($2 USD should be considered a minimum). The reasoning is simple, and twofold.
First, valet parkers tend to pool, and then split, their tips on a predetermined cut. Thus, the valet who parks your car may not be on the same cut as the valet who retrieves it.
Second, and more importantly, you are paying for the convenience of having someone park your car, and the convenience of having someone retrieve your car.
If someone doesn't want to pay for the convenience of having someone park/retrieve their vehicle, then they should utilize the nearest self-parking area provided. Simple, and to the point.
Now, let's talk about the "stiff". A stiff is typically one of two types of people. The first is that person that just doesn't know better. They believe the tip is already a part of their current "service" charge, or for some other reason of ignorance. Annoying, but not nearly as bad as the second type.
The second type of stiff is the person who utilizes the convenience of the service, but feels that they should not have to pay for the convenience of that service. It should be noted that the second type of stiff not only annoys the valet, but there may also be another customer, who is willing to pay for the service, who is now having to wait for their own vehicle immediately following the stiff.
Obviously, the second type is completely in it for themselves, and could careless about what is considered to be "fair."
So, what does giving a tip get you? If you tip up front, the valet now knows that their efforts are not in vain. They will provide you with the closest spot available under the known circumstances. Generally speaking, parking spots are first come, first serve. If all the close spots are already taken, then your vehicle is going to be out further.
However, a larger than normal tip can influence a valet to make an attempt to park your vehicle in a better spot. Worried about an upfront tip not being noticed later? Don't. Usually upfront tipping is documented in some manner by the taking valet.
Tipping also influences the valet to retrieve your vehicle as quickly as possible. Most professional valet employers hire valets to park and retrieve vehicles. It is generally not required in the job description for a valet to run to, and run from, the vehicle in order to accomplish the required parking and retrieving. Is a walking valet frowned upon? Yes. Required? Generally not.
Now, some valet pet peeves:
Kill switches, alarm systems, and unique quirks: Tell the valet of any such systems, or quirks, on the vehicle. More often than not, the valet will figure it out. However, it consumes unnecessary time.
Conspicuous mileage checking: Yes, even I've heard the horror stories of valet driving a vehicle around town ("heard", not confirmed). However, it has to be a rare event amongst professional valet parkers.
The reasoning of this occurring rarely is simpler than one would expect. Valets typically don't work alone, and they split their tips. To have a rogue valet joyriding is going to be noticed by other coworkers, and would be frowned upon, at the very least. Tips are earned by good customer service, and to endanger that source of income is not going to be left unchecked for very long. Even a lone valet will eventually be noticed as "missing" for any extended length of time.
If one must check mileage, do it discreetly and accurately. Nothing pisses someone off more than being accused of something they did not do. Plus, if it really does concern you that much, why risk valet parking in the first place?
Damage checks: This is more understandable than mileage checks. However, be absolutely certain the damage occurred by the valet. It drives me nuts knowing that people check their vehicle for door dings only after valet parking their vehicle, but almost never after leaving the local supermarket. So, how would one truly "know" the valet did the particular damage? It's always better to check your vehicle for damage immediately before you let the valet take it, not just after. And unless your vehicle is garage kept, not driven on a freeway/highway (where small rocks and debris chip paint), or not left in parking lots regularly to be door-dinged, don't blame the valet for very small damage unless you are absolutely certain it was the valet.
Above all, don't valet park a car with a known safety issue! Just because you may be willing to drive around with bad brakes, a broken steering column, coated scum on a windshield, or a vehicle that stalls regularly, does not mean a valet will want to take the same risk.