I don't really think there are any great tricks when it comes to car rentals and timing. You'll definitely want to be conscious of the fact that pricing is done on a 24-hour basis, so slight differences in pick-up and drop-off time can make a difference, but only sometimes. For example, if your flight lands at 1:00 pm and you plan to drop it off at 2:00 pm a week later, there's a possibility in some cases that the extra hour at the end will lead to being charged for an extra day (not always though). I'd play around with those things a little bit, to the extent that a change is realistic for your itinerary.
In the end, you want your vacation to go smoothly, so the best bet is probably just to reserve the car for the most realistic times possible. It's nice to have the peace of mind that the car will be there when you want it. Even then, there are times I've arrived at an airport at the exact time reserved with no car available. Plus, the reality is that car rental employees generally tend not to have the same level of customer service and eagerness to please that you find on Disney property. So in situations where you'd expect a Disney employee to bend over backwards to help you check-in early, a car rental company may not.
The only situation where I'm sure you wouldn't have a problem is if you drop your car off early at the end of the trip. They'll be happy to take it back early and have it back in the inventory to rent to someone else! (Just don't necessarily expect to get any money back for your early return though).