I was searching for info about the need of updating my Greek driver's license to international, but I can't get a clear answer about whether it's needed for use in the US. A friend told me that he's recently rented a car in the US with his Greek license and he never had a problem. Does anyone know for sure?
This page is helpful.
In general there's a very simple rule as to whether you should get an International Drivers Permit or not.
If your license is in English, or in any of the official languages of the country that you're visiting, then you do not normally need an International Drivers Permit.
If your license is NOT in English, or in any of the official languages of the country that you're visiting, then you would be best to carry an International Drivers Permit.
That's not to say that you'll necessarily need it, but even if you are able to rent a car using your foreign license, it's likely that if you were pulled over by the Police for any reason that they would not accept it if it's not in English.
The other reason for getting an International License is if your normal license does not have a photo on it. Most countries/states, including all in the US, have licenses with photos of them, so a license without a photo would likely be treated with suspicion.
At the end of the day, the laws are important here - but more important is how you are likely to be treated. Even if the letter of the law doesn't require it, it's still a good idea to carry an International Drivers Permit if any of the above conditions are true.