If you are booking in advance and not going during a peak holiday season, you should be able to get the AAA discount. I believe it is 20% off rooms rates, during value and regular seasons and 15% during peak season. If you only want to reserve a room, you can make the reservation yourself by call Disney directly. If you want to book a package, you'll have a TA at AAA make the reservation and then you will go through the TA for any request.
Ideas -
Call AAA, explain you are thinking about buying a membership for this purpose and ask if they can tell you if there are rooms w/AAA rate available for your dates and your resort choice.
Or call Disney... you will need to show the card at check-in but I'm not sure if it is required at the time you make the reservation.
You can only apply one (1) code to your reservation and can usually make changes as long as there are rooms available for your resort and dates.