The finish on the outside of most wooden stocks is fairly impervious to moisture. Oiled finish, or maybe urethane...
But look at the inside! Commonly, it's bare. So, if you wax it thoroughly, you'll make the whole stock fairly impervious to the warpage from a change in moisture content.
The high-quality, more expensive walnuts are denser, and thus more resistant to the effects of moisture.
I've never walked around in the rain while hunting, although I've been out during mist and drizzle. A one-day or two-day jaunt won't really bother things, generally. Going from a dry home-climate to a week-long hunt in very wet country might be another story entirely.
FWIW, Art