The tip is included in the price of the meal. If the service was decent and I didn't wait too long to order or for my food I tip decently. Plan on tipping, about 10%.
If there is an extra waiter or a bus boy leave some small change on the table for him or her too. You do not have to tip.
Greeks will sometimes leave a pitiful amount for a tip but that's harder to do with the euro for some reason. French and English people are tied for being the worst tippers, Germans are a close second! Come on! FCOL!
Food Is generally safe in mainland Greece and so is the water.
On Islands ask about the water.