In the US, if you're going to tip less than 15%, you should not tip at all. You should leave nothing and ask to speak with a manager to explain why you won't. If the failure in service doesn't rise to that level, then 15% is the minimum polite tip.
Of course, it's your wallet so do what you think is best. But this seems like a reasonable practice, so I follow it. I've actually never had to speak to a manager about service at a restaurant, though.