Check Kayak.com and reserve whatever is cheapest and most reasonable for your rental period. You can cancel at any time if you find better rates.
Sometimes car rental companies drop the rates 1-2 weeks before rental, which you can re-book and lock in lower rates. Also, I've found deals for $10-15 per day on Hotwire.com, but you pre-pay the rental and can't cancel. But if you find a car for that cheap, you've already gotten the lowest rate, most likely.