I use a cold mist humidifier in DD's room, and in ours, all winter. It helps combat the drying effects of the heat & drier winter weather, and keeps her (& us) from getting stuffy. Living in Atlanta, its humid enough when the weather warms up that I only use it then if she has a cold or spring allergies, again to keep her nasal passages moist & flowing.
It depends on your individual situation, but for us, this is true. In NYC, most of the buildings have radiator/steam pipe heat rather than a central air system. We need to run a humidifier in our own bedroom all winter our we'll get dry skin and a scabby nose. We received a a Crane cool mist duckie that we plan to run all winter + when sick.