I've practiced quite a bit in the rain (especially recently). At my hunting yardages (inside 25 yards), I've never noticed a difference in flight with wet feathers over dry ones. I've practiced when it was raining so hard, that I'd never hunt in it.

If you're one of those guys who likes to fling long shots, then you might have to limited your distance in heavy downpours, but I wouldn't get too worried about it. When an arrow is released from a bow, much of the water comes off from the sudden forward movement.