In a restaurant, one should always tip, unless the servers are paid a regular wage, and not a tip-earner wage, or there were serious issues with the service.
I once had an instance where there was a serious issue with the service, and I didn’t want to leave a tip. I spoke with the manager, and explained what the issue was, and how I felt about it. He agreed with me that in this case, I should NOT tip, and there were no hard feelings between me and him. I probably can’t say the same for the server, there’s no way he was happy – I believe he ended up getting fired for what happened.