I should probably let one of the organizers answer this, but I'll put in my 2¢ anyway. We will be driving our own car to Savannah, and we do have some concerns. For one, parking in the historic area could be an issue. There are public parking facilities, but on-street sites are limited. Also, I don't know where you'll be staying, but several places - as at the Staybridge Suites where we have reservations - have a per diem charge for cars.
So these are a few of the things you should consider when trying to decide whether to rent or not. In addition to taxis, I understand Savannah has a rather good transportation system, including a fare-free shuttle in the historic district. Also, many places are within walking distance of each other.
In sum, you may not want or need to have a car, but don't depend solely on my opinion.