For the last fifteen years I have been taking muzzleloaders from the Arid Southwest to the Midwest Rain Forest. In fact, some of them have been built partially in both places. I have never taken a lock out as some sort of preemptive. Yes, the guns dimensions grow slightly when going West to East, but not enough to damage the lock mortise.
If the lock is fitted properly with adequate draft in the lock plate, you will not have a problem.
The only caveat I would give, if you are spending a week outdoors camping or hunting and your gun gets drenched in rain or snow. Do not take it apart until it slowly dries out. If you have shot it, clean the barrel and lock in place.
The only way to make wood impervious to moisture is to use an epoxy finish, such as Thunderbird bow finish. All surfaces and mortises and the ramrod hole have to be finished with it. I don't do that.
My .54 caliber Hawken has the barrel channel bedded with epoxy, and the stock is finished with 5 coats of spar polyurethane. I can take the barrel out for cleaning. It has been in snow and rain for days on end. Not my favorite finish.
One other thing that will give you problems is a Hickory ramrod that hasn't been properly tapered or is tight in the thimbles. Hickory likes to swell.