As someone who's waited tables for over ten years, I've got a lot to say on the subject of tipping. I even once put together a sizzling "expose" on tipping ettiquete for a "news" show in college (please, please let that never resurface). To avoid going on a rant, I'll just say, tipping is not optional. Servers, at least in Michigan, are paid $2.65/ hour. Tips are not "extra" money, they are a server's paycheck. It can be argued that the service industry should change its practices and pay waitstaff at least minimum wage-- but until that happens, tips are servers livelihoods.
All that said, I am always shocked to hear about celebrities who tip poorly, or worse, not at all.