It's your money, therefore it may be expected of you, but it is not necessary. If you feel that you have been adequately service, then leave an adequate tip (15% is probably fine). If you have been serviced excellently, if you are willing and your budget allows, then tip a bit extra. If they did a very shitty job serving you, well than I have an anecdote for that.
Some time a few years ago, my father and I were in a restaurant which serviced us terribly. They forgot about our order, and came by our table around an hour later to take our order and offered no apology or compensation, and instead, they were quite rude to us. My father had just bought me a giant penny that was about as big as my hand as a souvenir from the place that we had visited, so when the tipping time came around, that's what they got, a giant penny with no monetary value whatsoever.
So yeah, you don't have to be a dick like we were when we received horrible service, but don't waste your money on people who won't even bother to service you properly when it's their job.