I wouldn't worry about rain but I sure would go prepared for it. I lived in Flagstaff for six years and recall that the first real storms of the winter often arrived sometime in November. In Flagstaff and on the canyon rim it would be snowing and down in the canyon it would be raining. November can also be a dry month on the Colorado Plateau. You need to plan for either likelihood. The canyon is beautiful in the rain.

One other thing, snow on the trails near the rim can make the footing treacherous so be sure you have good boot soles; a lot of people take light-weight instep crampons for this possibility.