When I used to tip I would tip O% for terrible service, 5% for poor service, 10% for average service, 15% for good service, and 20% for excellent service above and beyond the call of duty. On average it would work out to 15%. I strongly feel that if you are going to tip, you should let the tip serve its purpose of providing feedback to your server, rewarding or withholding a reward when appropriate. I don't understand this custom of blindly tipping a set percentage regardless of the quality of service.