Yes shafts can warp because of humidity. Moisture also exacerbates other causes of warping that you have no control over including the proper aging process or lack there of before & during manufacturing process. Shafts also can have grain pattern stresses some of which can be hidden and over time they will warp. Cue makers mitigate these reasons with their wood selection and manufacturing processes. Control you have is keeping the shafts straight with proper storage out of the sun and mitigating extreme temperature changes, There are stories of people keeping cues in their cars with absolutely no problems but why take the chance.