I suggest using mousesavers.com to check for codes at the various rental car agencies that you would consider using and then going to the those companies' websites to find the best rate currently available. When you go to mousesavers.com, click on "Plane, Rental Car, Shuttle & More" in the left column, then click on "Rental Car Discounts & Advice" again in the left column and then click on the individual companies names on the right to get to the specific discount codes. If you want, you can go ahead a make a reservation now to ensure that you won't pay more than the best rate you find today. However, you should keep checking the websites periodically (maybe once a week for the next several months and then more frequently beginning in late summer/fall) to see if the rates improve for your dates.
Usually a month or two out, the rental car agencies will forecast their position for a given week based on several factors (historical rental data, the number of pending reservations, etc.) and adjust rates as necessary. If there are fewer pending reservations than they want, they will lower their rates to drive business up. If they have a lot of pending reservations, then they will raise prices due to the principle of supply & demand.
Either way, they will NOT automatically lower the price on your reservation. You have to take the initiative to keep checking for lower prices. If you find a lower price, you can either 1) modify your existing reservation or 2) book a new reservation at the lower rate and then cancel your old reservation.
It's really a gamble. Sometimes a great rate is available on a rental car companies' website using a certain code for just a 24 hour period, then the next day the rate goes back up. If you happen to be checking rates that day, you're in luck! That's why I check the DIS boards Transportation page everyday to see if anyone finds a great deal on a minivan in Oct!
Hope this helps.
