I do believe in tipping, is the answer to Lise's original question. It's customary here. It's expected. If it wasn't customary and expected, then one wouldn't have to feel bad to not leave a tip. But generally, I always leave something, even if it's the worst service ever imagineable, which I have not encountered yet.
I've waitressed before and it's not easy being a server, and a pleasant one at the same time. I give credit to servers who try their best after long hours on his/her feet. I've never encountered a table who left nothing behind. If I did, I'd feel bad and would wonder what went wrong all evening. Getting a nice tip made me feel appreciated. It says "Thank you, I enjoyed my dining experience with you today."