If they actually have the nerve to ask for a tip, my response is 'when I get some service worthy of a tip i'll leave one'
I have on one occasion even asked for the 'service charge' to be removed from a bill as there was no service unless you think having a ladle of soup in my lap service.
However if the food and service has been good I generally leave a cash tip so that the waiter/waitress actually gets the money, if they want to share it after then it's their decision. However if it is added to the bill it usually means the company is going to use it to pay the staff their normal salary and it will not be a bonus. I object to this, if the restaurant wants to employ people they should pay them and keep their hands out of the gratuity 'Trunk'