The big danger is in sudden changes.

I have 5 solid-body guitars, and two electric acoustics. Where I live, (Calgary, Alberta), the air is fairly dry to begin with, and it gets even dryer during the winter. Then one of our famous Chinooks sets in, the temperature rises dramatically, and the humidity climbs by 10-20% over a four-hour period! The solid-body axes cope quite well with this flux. Being inside, in their cases, helps to cushion the impact. The electric-acoustics flex a bit. the action can rise or lower noticeably. I've been here for three years, and I haven't found any damage on any of my guitars. I recently sold a 1982 Daijon acoustic 12-string that I brought with me from Ontario: it made the transition very well, without any need for setting up.

Conclusion? Protecting your instruments from sudden changes is likely to be sufficient. I avoid using in-case humudifiers because the instrument has an environment in the case that may be very different from conditions outside the case. For home use, this may not be a problem, since they only come out for an hour or two at a time. Taking a guitar out of a humidified case at a gig and leaving it under the hot lights on a stand for four or five hours sounds like bad karma.