Depends.
The most important factor is your vacationing style. I will be staying at OKW in a 2 bedroom in January. I feel like I need a car because we plan on eating several meals in the room and need the car to get to the grocery store. Obviously if cooking on vacation is not your style (we enjoy it) then this isn't a factor.
Also, the time of year is an important factor. In January, I am not as worried about crowd size and plan on using Disney transportation between the parks and Downtown Disney. However, if the extra couple of minutes that it will take to use the buses is that precious to you, then by all means get a car.
There are many other factors, but as I said before, most of them depend on your specific vacation.
Why do you think you MIGHT need a car?