Well, I'll start in on this. I certainly don't have the answer but I do have my own opinion and decision.
I live in an area that can see humidity changes from 20%-90% over a 12 month period. Now, that's an outside measurement. Inside, It's more like 30%-80% with the average being 50-60%. Still a lot of room for disaster for guitars.
I don't therefore build to the average as that would be 55% which would lead in the low season to cracks for sure. I tend to follow the industry and stick to building in the 42-47% range and in the low season this is acceptable and unless I expose my guitars to more extreme low humidity would I worry about crack potential.
I also do not keep my guitars out of their cases. I feel that this also helps to maintain more consistent RH for the guitar and if I had a longer season (mine only lasts 30-45 days at most) in the low RH zones, I'd invest and use some humidity control devise in the way of a soundhole devise or one that fits in the case.
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