P.S. No sense trying to keep every guitar cased with or without humidor, as something has to be handy to take down off the wall and put back continuously.
How's that really more handy than taking it out of a case? How many fewer seconds does it take?
You ask a good question about how to prevent the very problems caused by keeping the guitars out, but you don't want to hear the best and simplest solution to the problem.
It's like addicts who say, "I want to be healthier, but I won't stop shooting up, so don't tell me to." Well, OK.
Wood is hygroscopic. I absorbs moisture when there's a lot of it, and releases its moisture when there isn't. The only way to keep a guitar at a given humidity is to limit exposure to the changes in humidity and temperature that take place in all rooms, and control its environment.
The temperature and humidity in every room varies over a 24 hour period, unless you want to deal with water bucket humidifiers, which I found are a giant PITA, YMMV.
On the other hand, keep a guitar in a small, controlled, environment most of the time, and it's much less apt to react to external humidity and temperature changes. A case is such an item. It buffers the temp and humidity changes.
Throw in a very good system that both absorbs excess moisture and releases moisture to the guitar when it needs it, and you're done, no hassle, no mess, no water buckets. In fact, PRS recommends it.
So there's a real solution, but it involves The Case That Was Invented To Protect The Instrument.
I put a D'Addario humidipak in the cases of all of my guitars, electric and acoustic, and have for years. And then I close the case with the guitar in it and replace the packs when they dry out. Unlike the old fashioned sponges or plaster of Paris stuff in plastic boxes, hey don't leak or smell.
I have not needed an adjustment of any kind on any of my recent guitars, including ones I've had for over a decade, and I'm in Michigan, where it's humid in summer and dry as bone in winter with forced air heating (even with the whole house humidifier I use).
You read that right: Not one setup or adjustment in ten years.
Having spent $5,000-10,000 on these items, I'm willing to undertake the very non-strenuous action of actually opening that case and lifting the guitar out instead of taking it down from a wall.
The benefit is obvious. Or you can do what you do and have the thing adjusted a few times a year.
If you went the route I do, which I don't expect but am posting this as a public service, you can save a buck or two with Boveda's brand, they make the stuff for D'Addario. I stick them in the D'Addario bags. They make a specific one for wooden instruments that's labeled 49%. It's perfect. I get them on Amazon.