Like others have said, there are no guarantees of AP discounts.
But the chances are pretty good.
For 2005 and 2006, the only dates that didn't have any AP discounts are:
10/5/05 - 10/15/05 (this is when they switched from AP rates to the Best Rate Program and because of how far in advance you had to book, these dates weren't available)
12/26/05 - 1/2/06 (christmas to new years day)
4/14/06 - 4/23/06 (weeks around Easter)
6/4/06 - 6/20/06 (peak summer ??)
12/24/06 - 1/2/06 (christmas to new years day)
This is based on the info at Mousesavers:
http://www.mousesavers.com/historicalwdwdiscounts.html
They don't include the discount amount, but from what I remember over the last 2 years, the average AP discount off of rack rate has been about $25 on values, $40-$45 on moderates, and $80-$100 on deluxes.
If you like to eat a lot of table service, the DDE can save you a lot of money. In addition to table service meals, it can also be used at several counter service locations at AK and it can be used at the food courts at the value resorts. I think you can get the DDE discount at HDDR, but only for the 9 pm show.