Geoff wrote: " (Top classical players take humidity control very seriously for their violins)".
Well, for one thing, when fiddles/violins/guitars become TOO dry then damage will occur to the instrument. (When they become too wet, there will also be problems.) Of course the wood used to make a violin has ideally dried for years if not decades in the makers' workshop. However, that is a different issue from humidifying the instrument when it is stored in it's case, especially during the winter when the air becomes very dry in heated houses.
If you "humidified" an accordion during the winter the way a fiddler or guitarist humidifies their instrument in its case, you might actually end up with some moisture-related problems in the reedwork. On the other hand, if the leather valves inside become too dry, that could also be a problem.
If I lived in an especially damp climate I would consider putting something in the case to suck up moisture, but I think that this shouldn't be done unless absolutely necessary.
So my answer is: don't screw around with this stuff and your accordion unless necessary; it is, in more ways than one, NOT a fiddle!
I had problems a couple of years ago with the cherry frames (the outer frames, not the bellows frames) of my accordion twisting a bit in the very dry conditions of the Yankee heated home. This resulted in air leaking around the gaskets.
This year, I have placed an instrument humidifier in the case, and I indroduce water occasionally. This is not a scientific experiment as I won't have before and after photographs of the reeds, but so far the wood seems happy.
One Louisiana accordion builder on this forum recommended never adding humidity to an accordion's environment. I understand the concern, but I'm not sure someone from Southwest Louisiana can really understand how dry things get: when it's 5 degrees F outside, static sparks fly in the house! There is no way that the environment inside my accordion case is going to become more humid than the 80% or 95% relative humidity conditions under which the instrument was constructed (unscientific presumption, I know).
Anyway, I'm not recommending this, but I will report on the condition of my reeds when Spring rolls around.
This may make a difference with an internally mounted microphone. The vibrating reed is right up by the orifice providing an overriding level measured in db. Reflective waves do eminate from these holes as well, but are unnoticable to the majority of ears. The blend of harmonics from all stops open and playing in octaves really swamps out any reflective waves.
I age my violin woods for two years. The wood is supplied air dried as well. I stopped building guitars 10 years ago. Playing the bowed instruments and many acoustic guitars (not all!)change the tonal response and in almost all cases, for the better.