I don't have a whole lot of experience but i have had the best luck when it was cloudy, foggy, and lightly snowing but i'm sure a light rain would be fine too. If it was a real heavy rain i would stay home but if its an off and on showers and sprinkles i say go for it. I think predators feel safer when its cloudy and its not as hot so they can move further before over heating during the day.
another thing is rain washes away scent so if its comeing down good you might be able to call them across where you have walked to set out your decoy etc.