The critical concern is the relative humidity that the guitars experience over the long term and the degree and speed of change in that humidity. In other words, it should be held constant as much as possible and the commonly accepted 'right' level of humidity is 45%. Too dry and some guitars will experience body cracking and/or shrinking of the fretboard (resulting in sharp, exposed frets). How bad these problems can be or how susceptible the guitars are depend on such things as the guitar's style (e.g. acoustic or solid body), the materials and types of wood used and the individual guitar itself. There are issues with too much moisture too but, from what I've read, they are less common.
Many of the injuries guitars suffer are fixable but there's always the risk that they are not. Not only can a lot of money be lost but the years devoted to breaking in a beloved guitar can't be recovered.
I most certainly am [U]not[/U] the guy to talk to about it. ;) Talk to luthiers and GT's Stephen White.
"It takes a lot of devotion and work, or maybe I should say play, because if you love it, that's what it amounts to. I haven't found any shortcuts, and I've been looking for a long time."
-- Chet Atkins