I have had two sets in my garage one was a percussion plus that was in there for 3 years and recently my new pearl exr and I live in Miami, Fl!!!! Its super humid all the time except winter where the humidity drops to maybe 50%!!!! The only thing I have noticed is during the winter when the temperature flexes from day to day is the drums tuning flexes up and down.
When you think about them warping it would be hard with the heads on. There is a different pressure inside the drum pushing out but with the heads off and sitting sideways on the floor maybe in a couple of months.