This is kind of a deal for me too... Well, you did not cook the food or bring it to me (some places, you order first at the cashier (no actual waitress) and they then bring it to your table but you get no service (e.g. get up and get your own condiments, etc, this is a trend where I live)), but I am to leave a tip... Hrm, will this influence the cook?
I think it is more agitating when it is a service that really does not "need" a tip. I called in an order, I walked up to the counter (its a lunch counter, everyone calls it in), and the checkout display puts up a tip amount (usually large amounts like 15, 20 or 25 percent). Well, uh, you cooked it, I thought that was included in the price. After all the cashier just reaches behind them and grabs the bag with my name. Very strange proposition. Is the cashier working for peanuts and this is their wage, or is the store taking it, or perhaps the cook? Who knows...