It can be kind of a two part answer.... whether a rental car reservation is honored if you're an hour early, and whether a deal will be honored if you're an hour early.
First, a car rental reservation will certainly be honored if you show up an hour early... but you may not be able to necessarily get the car an hour early. If the location is heavily and tightly booked, they may not have a vehicle ready and hour before (vehicles coming in may have to be processed and cleaned before being rented out).
If the location has lots of cars available there should be no problem getting the vehicle an hour early. Just keep in mind that your rental starts with the time that shows on your contract as your pickup time. If you had a tight 24 rental (say pickup noon on Monday and return noon Tuesday, and you pick it up at 11am and Monday and don't return it until noon on Tuesday, you will be charged for that extra hour -- most rates are calculated on 24 hour days from pickup time to return time.
Second, a deal may still be honored if you're an hour early, but only if it doesn't interfere with the terms and condtions of the deal. For example, if you have a weekend deal that starts at noon on Thursday and ends at noon on Monday... say a weekend rate of $19.99 per day. If you show up at 11am and want to take the car, they will give it to you if available, but it would negate the terms of a weekend rate requirement and the location would not give you a weekend rate structure... you would be charged a regular daily rate for each day, because the weekend rate structure doesn't allow a pickup before noon on a Thursday.
Car Rental Policies
Making Car Rental Reservations