March is iffy. End of March can be surprisingly hot. Also depending on the year, March can be spring break time, so be careful about that. The first week of March is usually safe from spring break though, and is generally uncrowded and very pleasant.
The end of October is actually a pretty popular time right now. The Mickey's Not So Scary Halloween Parties, Epcot Food & Wine Festival and adding in Columbus Day holidays and this can be a crowded time.
Better choices, IMO, would be early November (before Thanksgiving week obviously), early December and January. All those times are still very pleasant weather and generally the lowest crowd levels of the year besides September (which is very hot IMO).
Beth, mom to older DD (8/01) and younger DD (10/06) and always missing Leah (4/22 - 5/1/05)