I seem to get about the same deal no matter when I book, but where I save the most money is with the Travelocity special deals. I put in my dates for a car rental, and then there's usually a link to click at the top of the page that has special deals. I have already reserved one for our trip in July this way, it's about half price of what I was expecting to pay for a larger vehicle.
And the other great thing about the Travelocity deals is that it's just a reservation, you don't pay until you get the car. If you find something better or want to change it, it's totally easy to do.