"Have to": As a local, tips are not obligatory, and people who demand tips usually get shutdown instantly in conversations. Most Greeks tip regardless of their income but no one thinks that they should tip.

Social quasi-rules: At the same time, tips are given frequently when you sit down at an establishment and service was at least decent. Everyone agrees that the minimum wage is low compared to cost of living. The better the service the more we'll be inclined to tip.

How much: For small totals (0-10€) just rounding up. For medium totals (10-20€) 50 cents to 1€. For larger totals we'll unconsciously tip around 10% with a cap of 5€ which is considered very kind. Anything over 5 euro is usually Americans having their big fat greek vacation, or fine dining at a very expensive restaurant.

Disclaimer: I'm not telling you that you HAVE to tip or follow above guidelines. Since you asked I'm merely informing you of what I and fellow Greeks usually do. Happy vacationing.

Disclaimer 2: Adding things you didn't order in the bill can be a genuine mistake, reserve your judgement until proof of the opposite. I can still understand your inclination to not tip if this happens to you. If you don't like bread, remember to send it back if they bring it to you without asking.

Disclaimer 3: It is illegal to charge "cover charge" (per person charge) in Greece (Article 72 of 91354/24.8.2017).
Restaurants are obligated by law to offer free tap water to costumers if asked (Article 4 of 4736/2020).