15% is just a recommended benchmark. No standard.
Edit: Following on wolf's post, who else do you/don't you tip? I always tip parking attendants a $1 or two, but ONLY if they pull my car up to the front. I've had guys throw me the keys and point to my car in the single-level parking lot (those fuckers on 3rd St. just north of Market). First they double the price for overnight parking (I guess that's reasonable), then they're jerks about it.
I give cab drivers either:about $1 for every $5-8, or about a $1 for each person if there are a whole bunch of us (depending on the length of the ride, of course).
The worst are coat-check girls and the guys in the bathroom that hand you a towel and give you soap. I only tip those dudes if I take a cigarette or some gum. Sometimes more if I need a condom, but that's only been once - I generally try to be prepared at all times.
