Buongiorno,
If you are arriving to the airport and going directly to SG - then yes I would suggest getting the car right away. If you are staying overnight in Florence before heading out then, I suggest you wait. You can pick up the car the next day or the day of departure from Florence - even in the city center of Florence.
This article offers some pointers about renting a car in Tuscany.
http://www.discovertuscany.com/tuscany-car-rental/
Buon Viaggio,
Donna Denise
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