As long as tipping exists, you should always tip your server 20 percent, unless the server literally tells you to fuck off. Then you should question what the hell is going on with you, then still probably tip 15 percent (and leave). I once had a server knock over a large glass of ice water directly onto the front of my person and throughout my lap (at fancy Cafe Juanita, on my birthday); we still tipped 20 percent, because accidents happen (the restaurant didn't comp dessert or anything, which I would've if I were them, but whatever).
Servers work hard, and your tip is a large part of their pay—in some restaurants in some places, it’s all they get paid. Tip well or stay home.