People will tell you (accurately) that most Italian restaurants include a service charge (15 percent) and a coperte (or place-setting) charge, usually €2-€4. That said, the service charge is often never seen by the waiter. Good owners divvy up the spoils, frugal ones don’t. That’s why you can’t add your tip to a credit card bill — no one knows where those funds really ends up.
Since there’s so much going on between the lines it’s a good idea to look at it in North American terms, which is to say 15 to 20 percent on top of the bill. Italians will think you’re crazy, but generally they’re cheap (some leave one percent or less, or nothing). If you’ve been treated well, give back.
No one’s going to grouse at you if you don’t — not to your face, at least — and if you’re on a tight budget you can choose to leave little. But do leave something. And don’t sit around with pen and calculator trying to figure it out. Not kosher.