Well, as with everything else, there is a lot of debate. But humidity does affect all animals, including humans (in my case is mainly my sinuous and skin). For gliders their most sensitive part is their ears, low humidity can damage them. Below is a picture:
And as long as it is not dripping wet, the humidity should be good enough. As I said, for my own health I try to keep it around 85-90% and I've never noticed them having any issue, not even during winter.