I've always had the best luck with Alamo at a week and a half to two weeks before the date I need to pick up the car. Prices seem to bottom out right around then, when it gets close and they have a better idea of how many cars will be sitting around unrented. But don't wait too long, because they go way up again when you get too close to the date.
But I'd find something that won't nauseate you to have to pay, and keep an eye on rates a couple weeks before you leave and try and get a better deal. I've always gotten the best deal at Alamo when I wait til that 2 week mark and couple it with an Entertainment coupon.