If you wait the national chains will lower their rates the closer your trip gets. Check out the other threads such as June 2013 car thread. You will notice when this thread was started many months ago folks were quoted very high rates. But as their trip got closer the rates took a big drop.
I never book my car until the Tues before my trip. THat is when the rental companies release their last minutes deals and for my last 5 trips I have not paid more that $104 for a mid size weekly rental.
Thanks, Shelley. It's just always so nice to be able to have things booked early!