While we're not GREAT tippers, we do tip relatively well for good service. When we walk into a restaurant, we expect to tip 15%. That may get adjusted up or down, depending on a variety of factors. Quality of service, speed of server, quality of food, (yes, I hold the server responsible for kitchen screw-ups) etc. NORMALLY, we end up tipping between 15-25%, but there have been a couple of times where we left a mere 2 cents on the table to make a point. Never leave just no tip, that can be misconstrued as "they forgot", or just don't believe in tipping. Leaving 2 pennies makes the point that you were VERY unhappy with the service received.
We had a waitress follow us into the parking lot in a mountain resort town one time to complain about the tip. We had left (almost to the penny) 15%, and she didn't think that was sufficient. I asked her to show me how much I had left, and when she held it out, I counted it, and apologized profusely...and stuck it in my pocket. I think she expected me to take out more, but instead, I told her that tipping is not the law, but a gratuity for good service, and she only provided mediocre service. I thought she was doing good to get the 15%, but if that wasn't good enough, I'd just keep it...then, I walked back in and told her manager what had happened. Handed him the tip to spread amongst the bus staff, since they did much of her work anyway. (getting/refilling water glasses, napkins, etc.) He fired her on the spot. Apparently, it wasn't the first time she had done this...