How to tip in Greece

I always tip 10% at restaurants anywhere in Europe, including Greece. Not everyone does that but for me that is a reasonable amount. I generally don't tip for drinks or takeaway food (unless it is being delivered). I do leave a few euros per night in hotel rooms for the cleaning staff.

You can tip or not, it's up to you.
As you already know, service is always included. You can round up, for example, paying €40 for a restaurant bill of €37 or €38.

The tip for a private guide can be more substantial. But it's still up to you.

At restaurants and bars, when I pay by card, since I always have some cash, I use a few coins for the tip, which I leave on the table.

Some "smart" restaurants in tourist spots are now equipped with payment machines with tipping options, like in the US.
I've even seen this in a small street food restaurant. When I saw this, I didn't leave a tip, neither on the payment terminal nor in cash. And I told the guy I wasn't American.
So, personally, I highly recommend you do as I did.

Also note that the prices displayed always include VAT.

I don't like the tipping culture in the US. I don't want to contribute to it its spread to Europe.

I do as Jo Lui does.

I do the same …. And will start paying with cash if I begin seeing that machine.

You can just choose no tip on that machine.

Being Australians, we almost never tip, not here, not in London and not in Europe. We do tip when we have visited the US, but I dislike it so much that I probably wouldn't return.

We do sometimes round up, it depends on how good the service was.

To me we pay large sums of money for a restaurant meal, and for a hotel room and that includes the fact that someone is serving us and making our beds.

Well, well. Looks like so far you have gotten different advice on tipping in Greece. Imagine that.

We prefer to tip in cash, and hand it directly to the server rather than leaving it on the table. That ensures they actually receive it.