My experience is that unless you are booked over a very busy time (holiday weekend, Xmas, etc.), car rental rates WILL come down as you get closer to your date. On our last trip, the lowest rate we found was 48hrs before we left (then it went up again 24hrs before we left). I would keep watching. Start watching now every couple of days, and then 3-4 weeks before your trip, start checking daily.
This trip, I saw the rates drop at exactly the two week mark, then they went up again a couple of days later. We leave in 1 week, so they may drop again or may not - but then, I'm happy with our current rate ($231 including taxes for a minivan for 10 days) so not knocking myself out to find something cheaper.