Acoustic or electric guitars? Acoustic need more care than electric.
Has more to do with hollow body vs solid body than acoustic or electric. It's gonna take REALLY dry conditions a really long time to do anything bad to a solid-body other than give it a bit of fret sprout, which is easily enough remedied. But really dry or really wet conditions or rapid swings in humidity can play hell with a hollow body guitar, whether it's got wires and pickups or not. The top, back and sides can all warp, the top and back can crack, etc.
In the coldest part of the winter with the heat blasting, the humidity in my place will get down into the mid 20% range, even with a whole house humidifier trying to counteract it - it'd probably be in the low teens without that. I have a couple of small evaporative humidifiers that do a decent job, but they'll just get my man cave / guitar room up to maybe mid-30% neighborhood and that's running pretty much full time. BUT, they'll get the humidity in their immediate area up to the mid 40s, which is about perfect for my one guitar that I'm concerned about, a Martin acoustic. So I put them both pretty much right below where it hangs and the meter I rest on the Martin's shoulder shows humidity around 45-48%. I wouldn't do this with a misting humidifier, but with just an evaporative one, it's no problem. When it gets crazy humid in the summer, I generally have the house closed up and AC running, which brings the humidity down into the right range. And in the Spring and Fall, with the windows open a lot, it usually doesn't get too high or low. If it's just too extreme, I just stick the Martin in it's case with Humidipaks, which do a great job of bringing the humidity UP or DOWN to around 45-50%.
I'm playing around with an Ibanez Casino knockoff at the moment that's completely hollow except for a block to mount the bridge to. If I end up keeping it, I'm gonna have to worry somewhat about it too, although with a total of $300 invested in it, I probably wouldn't worry too much. Except it's a shockingly nice guitar and they don't make 'em anymore, so I'd probably try to keep it in the right range as well...
-Ray