I don't know about March, but my boyfriend and I went for 5 days the last week of May during our last trip in 2009, and the crowds were totally manageable. We were there during grad night, but we just left the parks early that day and did some shopping in DTD to avoid the larger crowds. However, I know that school ends much sooner than it used to around where I live (the first week of June or last week of May depending on the district), and I don't know if the same thing is happening in Anaheim/LA. If so, the crowds might be a bit heavier than we went. It might be a good idea for you to find out when the local schools are done for the year if possible.