Unless there is lightning involved, it isn't a question of whether or not go go but rather a question of what to wear. I'm not sure if fish are more or less apt to take the fly in the rain but I know there are fewer people out whipping up the water. The increased disturbance in the surface has to make us less visible (at least the part of us above the water). I personally go with wet flies and nymphs in the rain on the unsubstantiated idea that a dry fly on the surface in the rain is less visible due to the chaotic surface disturbance the rain creates.

At any rate, I know I've caught fish in the rain, so it's always worth going. Watch for rising water if the rains get heavy at times.

And a very broad brimmed hat can help with that cigar.