Depends on what you have in mind for shopping, really.
If you plan on hitting an outlet mall or something like that, then a car would certainly be handy to have. ; Shopping on-site is pretty much all Disney Themed stuff... and priced accordingly!
Once you're on-site at Disney, you don't really need a car to get from park to park... the Disney transportation system is pretty effective... you just hop a bus, boat, or monorail and you can get pretty much anywhere on-site without much difficulty (you might have to transfer routes, but it's generally straighforward and the drivers/operators/platform attendants are pretty good for helping you get where you want to go)... that's for all the resorts, the Magic Kingdom, Epcot, MGM, Animal Kingdom, Downtown Disney, and the two water parks.