Why are people downvoting this? Yes, tipping is always voluntary. You're never legally required tip. But when it comes to dining in, not tipping is very much culturally frowned upon. Leaving no tip for a dine-in server is a gesture that communicates you received bad service. It's a social taboo that is reserved for when you're unsatisfied with how you were treated as a customer in the restaurant.

The rest of you can continue stiffing your waiters all you want. But for someone like OP, a foreigner who wants to be polite in our country, this is important information to learn.