I keep hygrometers in my cases and on the walls in practically every room where I live. When the Santa Ana winds come through here the humidity can drop very low very fast. When that is predicted I use a damp sponge that is contained in a ziplock back that is perforated with tiny holes on one side. I lay it in the area underneath the headstock. I try to keep my instruments at a constant 50% humidity, and have never had a problem with warping or cracks.