I worked in the bar and food service industry for a long time and not once have I asked for a tip. I don't care if I gave stellar service and got stiffed I just bit my tongue and remembered the customer next time they came in.

If she had done that to me I would have simply told her that was left was in direct proportion to the service we felt we recieved and if she wants more then next time the service has to be one step better. Of course I also would have pointed out that she would not get that chance because I would not be returning and that she needs to get her manager that very moment.

I hate the way tips work now, waitstaff assumes that simply by being there you owe them a gratuity. I hate like hell stiffing a waiter or waitress but I have done it more than a few times due to very poor service.