Well, I tip for lots of things, too.

I always tip my hotel maid (sorry if I get that one started here). I just think it's appropriate, and part of it is the outrage that doormen pull down these huge incomes while housekeeping subsists on a bare minimum. But mostly, it's that my room being kept tidy during my stay is a personal service for me and me alone.

I also almost always tip valet parkers, even if there's no other option. Although, I had a recent exception to this. My health club periodically has special events and valets take over the lot. I don't tip then, figuring that they are included in the annual holiday fund tip. I also didn't tip when I lived in buildings in Chicago that required valet parking. Again, they were covered by an annual tip (and they beat the hell out of the cars on a regular basis--tip or no tip).

I assume a 20% tip on table service as the default value (but it may go up or down).

I don't tip the random little tip jars that have taken over service counters these past few years.

In general, if I receive a personal service for which tipping is customary, I'll tip and do so generously. But I also avoid unnecessary and unwanted tippable services like skycaps when I can.