If you got into a CS restaurant, there is no tip. You just order what you want and pay cash, card or with your room charge anything extra.
If you go to a TS restaurant and order things in addition to your dining plan option, they will bring a seperate bill for you at the end and it includes the 18% tip. If you're talking about a $5 side dish, the 18% is not much anyways. You are always welcome to dispute the added gratituty with management.
The ONLY time we had extras with the dining plan were beverages. It's tons of food already. We ordered alcohol several times and we split meals and ordered a drink. Many times, if we just ordered an extra glass of tea, our waiter didn't bother ringing it.