We'll be there when you are, too! (We're starting our stay at the AKL, then moving to the Polynesian on 3/16.)
We've been several times in mid-March (our Spring Break.) There are usually lots of people, but it's not unbearable. We take a pretty relaxed approach to touring... we use Fastpass, and we skip any attraction with much of a wait (and come back later... like during a parade). As long as you're flexible, you can see and do as much as you have the energy for this time of year!
We're not usually early risers on vacation, so I try to avoid the park that has Early Entry each day. The parks seems to get the most crowded in the early afternoons (around noon - 2), so we often take a break or switch parks then. We'll often have a late breakfast, then a late lunch (like around that 2:00 window)- having your meals a little later than the majority helps with the crowds, too.