It is ok to leave a humid hide in the enclosure all the time.

But from my experience, I would pay close attention to the amount of time your bp is in there though. Since ball pythons like to feel snug in their hides, I have noticed that sometimes when a humid hide is in the enclosure all the time...the bp will prefer to stay in the humid hide because of the snug fit provided by the stuff inside the hide. I did not experience any problems associated with keeping the humid hide in the enclosure all the time. However, I would definately make sure that the damp material (paper towels or moss) inside the humid hide was changed every day. The frequent changing of the material will prevent bacteria and mold from growing to levels that can cause problems.