Reserve the car. If you end up not needing it, call and cancel as soon as you know. Even if it is on the same day, that's okay. You have a reasonable expectation of needing it, and I'm sure will do your best to find out earlier rather than later, and to call as soon as you know one way or the other.
The agency policies are written to enable this when it is needed. They allow for expected no-shows by overbooking, just like airlines do, and just like the Disney restaurants do---at least, those without a no-show penalty. As long as the historical no-show rate doesn't change drastically, they will be able to accurately predict demand.
If the no-show rate becomes unpredictable, or the degree of overbooking gets to be so large as to be risky, then the rental agencies will begin applying some sort of no-show penalty. A few might also do it for tighter yeild management. But, in the grand scheme of things, a few leisure travelers with uncertain plans are a drop in the bucket compared to the business travelers who routinely change plans late in the game.