Need to pre book taxi from airport to center Athens?

We have consistently used Welcome Pickups in several cities, including Athens. They have been excellent, and yes, you must pay online when booking.

The airport will have plenty of taxis available. It is a fixed rate (€40 during the day, €55 from midnight to 5am) to the city center. No need to book in advance.

Thanks for that. However, I wondered how much more we could be expected to pay for the. 4 of us? Is that too a set price or do we just negotiate with the driver?

My general advice is to know in advance if your hotel is available for drop off at the hotel door & then don't leave the taxi, whether Welcome Pick-ups or not, until you are at the hotel door. Don't accept "there's too much traffic" or any other excuse used to let you out at a corner. Don't tip unless you have you have the cash in hand when you get in the taxi at the airport. Our non-experienced travel friends were put out at a corner in to what they thought was set up ... lost iPhone and credit cards. Was Welcome Pick-ups in Monastiraki area . They didn't realize they weren't supposed to tip. Please stay aware of transitions and open purses.

Excellent advice. Texting hotel now.

Just to second travel4fun's answer.

You can, like hundreds of travelers every day, take a taxi at the airport taxi rank.
There are plenty of them. The fare is a flat rate, as mentioned. This flat rate includes everything, including baggage and tolls. It's indicated on signs at the taxi rank.

Taxis are allowed to take up to 4 passengers (in addition to the driver).

There are some unscrupulous drivers who may tell you that you have to pay more for this or that... It's a scam.
If you pay in cash, the other scam that can happen is when you pay, for example, with a €50 bill. While you're not paying attention, the driver discreetly exchanges the €50 bill for a €20 bill and pretends you made a mistake.

I've often taken taxis in Greece and never experienced this, and I've always had very friendly drivers. So don't look like an innocent tourist, make sure the driver understands that you know the fare.

On first contact, saying hello in Greek can help ("kalimera" for good morning and "kalispera" for good afternoon).

It's true that there are a few pedestrian streets in downtown Athens. However, it's very easy to locate your hotel with Google Street View to see if vehicles can reach it.

Thanks so much to the several people who responded in detail. Very good to know. We’re very used to taxis in other foreign cities but unfamiliar with Greece. Will do as you all suggested.