I think it depends on what type of casserole...is it cream based? Are any of the ingredients starting out raw? or are you combining some precooked or canned items? Again, ham type? How much liquid will you have in there or is it mainly cheese, ham and some veggies...and what are the veggies and how big or small are they cut up. Also, are you using ceramic or glass or silicone in the microwave?
Thoreau said, 'A man is rich in proportion to the things he can leave alone.'