I keep a humidity gauge where I store my guitars (in the basement no less). Whenever it dips below 40, I sometimes turn on a cheapo room humidifier. I don't recall ever doing it in the summer. In the winter, I turn it on a fair amount. I try to keep the levels between 40-60%. I'm not anal about it by any means.
A while back, some friends of mine that play where discussing instruments that had developed cracks. Our sample size was small (40ish as I remember). There where 3-4 instruments that had developed cracks. They had all been babied (faithfully humidified) and then developed cracks during a small window of time when they had not been cared for as they where acustomed.
Of the 40 or so instruments, probably 30 had sporadic humidifing - or none at all. Many of these instruments where old (20-50 years). Our hypothesis over a few beers was that having your instrument endure temperature and humidity swings early in it's life toughens it up. Faithful care of on instrument increases the odds of it staying crack free - but the moment the environment becomes less than ideal, it stands a greater chance to fail (compared to one of our toughened up instruments).
I've posted this on another board and had luthiers chime in that this isn't based in fact and makes no sense. I've also had others reply that they had noticed the same thing. /shrug.
I doubt I'll ever change my habits with instrument care. Some of my mando's have finish checking from trips to Telluride and other camp trips (hot and cold swings) - but nothing structual. Hell, my '58 Gibson A40 has lived in Texas for years, then spent 20 years in Breckinridge - now with me in Denver. It has never been well cared for and is totally sound.
Now, none of my instruments are worth more than $2k - so I'm not going to be devistated if a crack does appear. I may have different slant on it if I owned a pre-war martin d28.
Listening to:
the sound of the clock ticking closer and closer to 5p
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