We did not rent a car for our first Orlando getaway (where we stayed on Disney property).
All subsequent trips, we've rented a vehicle and have been much happier.
While Disney transportation was okay for getting around the parks, we disliked being held hostage exclusively on Disney property. We love exploring the offsite stores and attractions, and a rental vehicle gives us that freedom.
The best tip is to check out the rental car thread for your month of travel over on the transportation board, and subscribe to it. Then check it regularly as people will post deals, codes and coupons for your travel month.
Never lock into something early, and be prepared to cancel and rebook when prices go lower. Usually you will find your best deals about 6-8 weeks from your departure date.
When comparing rates, be sure to compare all the variables (is a spousal second driver free or an extra daily charge?, etc.).
And bring your own GPS in your carry-on. It will save you quite a bit. And a GPS is a must, IMHO!