Hello from Georgia,
I have hunted for extended periods in the rain with wood firearms and never experienced any appearant change in P.O.I. Now I have never taken a gun out to my range while it was raining to see if it would have an effect, but the deer that I have shot while raining (4 as well as I can remember) have all been shot exactly where I was aiming. I am sure that at some point that moisture would have an effect, but I believe that it would take a thorough soaking for some time for there to be enough difference to be an issue. I personally dislike synthetic stocks (fiberglass and "plastic") and have recently taken a huge liking to laminated stocks. To me this is the best of both worlds. Wood that is all but impervious to weather. My H&R Ultra .308 has a very nice laminated and checkered stock that really sets the rifle off.
Thinking of this post has me considering going down to my range the next time there is a long driving rain for some real life testing. I will post the results !!! Good Luck.
Dave