Thanks everyone. We fly into Rome & spend a few days there before getting the car to drive to Chianti. As Jean said, sounds like we can avoid some "schlepping" by just taking a taxi (or two) to Rome airport and getting the car. That way we can do some of the Val D'Orcia area on our drive to Chianti.

I would go to wherever you can get the vehicle you want. If I could get it IN Rome, I wouldn't go all the way to the airport.

I wouldn't go all the way to the airport either. Since you can easily arrive before 1:00 in Rome, I would pick a corporate location on the north side that rents vans so you can easily exit the city.

The exit from Rome for a trip to the north is equally easy from the airport as from Via Marsala (by Termini station). Any other pickup locations in Rome are on the small side and run the same risks as the small rental agencies outside of Rome. The driving time is close to the same, maybe 15 minutes longer from the airport, depending on the time of day, because you get right on the GRA. On a Saturday, there shouldn't be much of a problem with traffic, especially if you go early enough to miss the rush hour; rush hour is also a bad time to leave from Via Marsala. The taxi fare to the airport would be somewhat more than to Termini station, but the rental cost may be cheaper. The time getting to the airport would be somewhat longer than getting to Termini station if you're staying right in central Rome. If you're staying west of the Tiber, it would be about the same.

All in all, I wouldn't rule out renting at the airport; in my opinion, they're equally convenient, and it would depend on other factors, such as price.