Assuming there are no cancellation or rebooking charges, my advice is to book the best rate you can find now, but periodically check to see if rates drop before your trip.
Car rental rates can be tricky in Hawaii. When we booked our Aulani stay at the 7-month mark, we also booked an 8-day car rental with Alamo. At that time, the best rate we could find was about $470 (tax, fees included) for a midsize SUV. The rates for an economy or compact car were actually higher than the rate for the SUV. We kept checking the Alamo website as our trip got closer. Then, about 6 weeks before our trip, I noticed Alamo was offering a midsize car for $139 (tax, fees included) for the same 8 day period. I canceled my original reservation and rebooked the cheaper rate.
Good luck!!