We were there 2-1/2 years ago and woke up one morning to rain. We are from Seattle, so we were laughing at the people in their ponchos that we saw from our room before heading to the park...haha! What a bunch of wimps in their ponchos...we don't use umbrellas in Seattle (lest we be mistaken for tourists) so we were certainly not going to balk at a little SoCal rain.
So out into the rain we went. And then we turned straight back around and bought ponchos. It was POURING.
We went off to the park anyway and were soaked, and kind of miserable.
I can't advise that you cancel your trip or not, but I will say, it can really come down hard in Anaheim and we were not prepared for that. I think it depends on your tolerance?
The parks were practically deserted though, that was nice.