The wood shrinks as it dries out and swells as it becomes moist. Moreover, the woods that comprise most guitar tops and backs have quite different rates of shrinkage. The top will want to shrink more than the back and sides will want to allow and a cracked top will be the likely result. It is important to realize that relative humidity changes from location to location, from season to season, from day to day, from hour to hour. It just reduces its likelihood.
Wood is amazingly resilient, actually, and can even acclimatize over time.
But it can only do this if its adapting to a gradual change. It is these abrupt changes that create the greatest problems. You must monitor it every day. Quite the contrary. Certainly, if your skin feels dry, it's time to humidify your guitar - or, at least, to look at it carefully.