In a hard rain, they will get under pines and cedars. In fact, looking at the forecast for rain tomorrow I am planning on starting out near a cedar grove with just that thought in mind.
A very light rain though doesn't seem to affect them much, I'll hunt them as normal.
Between the two extremes of very light rain and heavy rain, they seem to prefer the open fields IMO. Other have observed, and I agree, that the sound of the rain in the timber is disconcerting to them and they gravitate to the open fields and rely on their eyesight as their hearing is muted by the sound of the rain.
They won't gobble as much during a rain, but they can still be had. I once called in a strutting tom in the rain that was so wet that his tailfeathers looked like sticks, the feathers were completely matted down. It was an odd sight.