I think that earlier is better - I remember reading that the heat gets unbearable and then there are lots of things like leeches (leaches?). I was there in around March/April years ago and it was fine - lovely in fact. I'm very envious.
I'm hoping to go to Nepal next year and want to fit it in with this years annual leave schedule and therefore go before the end of March. I could do two weeks at the endofMarch and a few days into April. My friend would prefer it later on.