Book the best rate that you can find now, and just cancel the reservation when/if you find a better one.
I am renting a mini van for a week in April 2015. The best rate that I have found so far is $335 (including all taxes and fees). This is with Dollar throuh
AAA.
I have also found Hertz at $450 through AAA and Avus at $450 with my company discount. The advantage of Hertz and
Avis is my DW can be added as a driver for free. I believe Hertz has other benefits like free child seats. (I think this is only if you rent Hertz through AAA and are AAA member).
Dollar charges $12 per day for additional driver and I'm sure will charge for car seats.
I am hoping rates drop as we get closer to April.