We went the first week of March this year (Wed-Fri) and it was great, crowd-wise. Went to the parks when we landed on Wednesday and DL on Thursday. It rained in the morning but cleared by the afternoon. The longest line was for Storybook Land Canal Boats (30 min). Everything else was very close to walk-on. Small World and Splash Mountain were closed for refurb. I would say the temp was in the 60s.
However, it was pouring down rain on Friday! We definitely needed the rain to help with this drought but frankly, it made the day at DCA amazing! It was completely dead (as everyone else was hiding in the Starbucks). Nothing was closed due to weather and pretty much practically walk-on for everything. Also, there was an initial crowd for RSR FP, but we went directly to Soarin and Muppets and once they were done, we walked right up and got RSR FP for 2pm that afternoon. We rode nearly every ride in DCA and were done with the park by 4pm -- just in time to rest at the hotel for a bit.
The one downside was that they cancelled the fireworks and Fantasmic, so we didn't get to see those.
I went over President's Day in 2013 and the crowds were astonishing.
I'm considering a March or May trip next year as well. March will be a gamble with the weather, but it has its benefits whether or not it rains.