My understanding is that between President's Day and end of March is a peak travel time at WDW. Translated: crowds and higher resort costs. Many schools across the U.S. have spring breaks that run through this time period. Because of this, rates tend to be higher in March, and I do know that there have been years past that discount codes have not been offered for this period. If you really want to travel in March, a cost savings tip would be to rent points from a DVC member. Rental points are going for about 10/point, which could be a cost savings over other WDW resorts (except maybe the value resorts). If you are not familiar with renting points, hop over to the DVC rent/trade board for more info. I personally have never been to WDW in March.
We have gone to WDW in May, however, and I think it is preferable to March for the reasons specified above. We were at WDW this past May (for a week around Mother's Day), and the weather was great (hot and no rain). We were able to get a room at AKL for $129/night. We had minimal to no waits for rides, with the exception of the MK on Spectro night (that place was a madhouse so we hopped over to Epcot, which was virtually empty). If you avoid Memorial Day week, I think May would be an ideal time to go.