As far as a humidifier, if you want to take absolute best care of your guitar, you should use one. I don't recall what guitar you have, but the more expensive, the more likely you will want to do this, obviously.
For me personally, I don't bother. I kind of demand a certain level of fortitude from my instruments, such as being able to survive where I do with no special treatment.
My dad has a Martin from the 40's. It has wear, to be sure, but nothing on it that I would say was an environmental issue. I remember playing with it like a toy when I was little. It was not enshrined or even kept in a case, but has survived quite well.
...so ever since then, I always hang on to the buckle.