well, I can not talk about your use case, obviously, but that's the general trend from the people I´ve spoken with. us Southern Europeans only have central heating as a common amenity relatively late compared to other places, so the most common situation was building quite dry in Summer (as the old Spanish makers used to) and then the high RH level would cause a potential problem. but I digress. the recommendation I get from most US builders is to build about 45-50%, while many guys around these parts will happily build at 60%RH (which I think is pushing it).
the thing with action being all over the place with RH fluctuations is a common problem IF you glue a high absorbing, high density and unstable wood like ebony with a stable, middle density and stable wood like mahogany. the ebony will swell more than the mahogany with higher RH and make neck backbone quite easily. I build guitars for a living and try to convince customers to avoid ebony like the plague, but most just seem settled on ebony because a black fingerboard is what a classical guitar looks like and a regular looking classical guitar its what they want. to compensate I always use carbon fibre reinforcement on necks, but never a truss rod - truss rods tend to be abused by owners trying to compensate for neck curvature, which is not their intended purpose.
but again, I digress. sorry about that.
