I say that because it seems from your response that you would tip regardless of how bad the service was. I agree with tipping but not with that.

I cant actually recall whether this happens in the US or not as most of my meals are drive-thru's.
A restaurant bill here will more than likely arrive with 'service included', usually 10 or 12.5%. If you notice it in time, you do of course have the right to amend the bill. However, when, as is usually the case, you just place your credit card with the bill, you will be charged an inclusive amount. Then, when the receipt arrives for you to sign, it will still have the space for you to add the 'optional' tip, or of course you can leave that in cash.

Would you tip on top of the service charge?