The ONLY park to worry about for crowds during (EVENING) EMH is MK.
All the others seem to have relatively low attendance.
In MARCH at (EVENING) EMH, you'll likely feel like the "only ones there" in MGM on several rides (especially as the evening gets later).
As far as MORNING EMH... they are nearly ALWAYS a great way to "do the attractions" at any of the parks, including MK... as many, MANY guests "will NOT get up early" on vacation... we LOVE morning EMH's!