Most of the Greek hotel stays include breakfast (BB) and fewer of them breakfast and a meal (HB) in the room rate.
A 13% service fee used to be included in the bill at restaurants, bars etc., but not since the late 80's!
An additional tip is is not obligatory, but it is common to round off the amount, especially when paying in cash. For example, you may choose to pay 20 Euro for a 18,60 Euro bill, if you are satisfied with the service. Such tips can be left on the table, or you may tell the server that you don't expect to be given any change. In general, a 10 to 15% tip is usually sufficient.
Try to avoid the places that offer "free" wine or have insisting aggrevators following you to the end of their restaurant lane, trying to convince you on their quality and prices. ALWAYS check their menu, don't stay on the page with the salads. Last tip, do follow the locals, or even ask them, as they know better.
Check to see if the bill agrees with what you've read on the restaurant's menu. The price quoted at the end of the bill is final, incl. taxes etc.
Tipping is optional keeping in mind most young waitresses/waiters rely on their tips as wages can be low in Greece, and as elsewhere, it is polite to tip for good service.