Like most other places, it's required you leave a tip at restaurants, diners and bars. However, this is usually taken care of by the restaurant itself. By law, restaurants in Costa Rica automatically apply a 10% service charge to your bill, which goes to the waiter. So leaving a cash tip at the table is not necessary, as it has already been taken care of for you when you paid. You can, of course, still leave a cash tip if you like, though it's not customary.