Tipping is a custom of west which I think was started to reward the people who serve for us in a little way we can. I usually leave a tip depending upon the quality of service but sometimes not if the service charges are included. I read somewhere that in Japan tipping is not accepted. If you leave a tip in some restaurant, the waiter will run behind and tell that you have left some money mistakenly.
We payed 20 Rs for his excellent service, he said "Saab Aapne to Bill me diya he na, Woh Kaafhi Hai Humko".
The amount you have paid against the bill is enough for us.