SoCal Etiquette Problem

So for the third year in a row, I've been honoured by an invitation to the Holiday party at the President's house. This is rather stretching my theory that they ask all the fancy folk, and then a few people like me along as seat-fillers (like at the Oscars) but, hey, I'm a string theorist, I stretch theories for a living...I don't see a better alternative theory, you see. Anyway, I need your help with a dilemma that comes up every time. I'm going on my own this year so I could go on my bike....but I'll tell you now I'm not going to cycle over there...it just does not work so well to cycle quite that far dressed up in one's fancier duds, even if it is in one of the swankiest neighbourhoods around. So I'm taking my car. I've made sure to get it nice and clean so that it is not too embarrassed near all the truly fancy bling-bling cars. It does not do to embarrass your car. It can take revenge at the most awkward of times. But in taking my car I end up with a problem every year. A very Southern California one. When you go to a function at a private residence, and they have a troupe of valets to park the guests' cars, are you supposed to tip the valets when they bring your car back to you? I've never got a good look at what others do when their cars pull up. Also, some of my fancy neighbours have done this for their parties (I'm still shocked by this) and I admit that I did get the binoculars out one time to see if I could see what the guests do, but I failed: Fell asleep before the guests started to leave - young people's party you see. Finishes way after my bedtime, and I need my beauty sleep, you know. Last year I did not tip, and felt particularly bad, as the very young girl who pulled up in my car (yes, I nearly had a heart attack - worrying about my transmission -as I do with all valets) handed me the keys looking all pleased, and (possibly) expectant. (It might be that she was just grinning at having won the short straw to drive the least fancy car in the lot...some sort of ironic reversal I hear that the hip young people go in for...) Anyway, your answers would be helpful. Tip? No tip? If tip, how much, since there's nothing to compute a percentage of. It's tonight. I must leave in four hours. -cvj