I work in the hotel industry and hotels management DO usually have a master key or master combination for safes. There are usually two-three people in the hotel who have access to this, depending on the size of the hotel, and its usually high level management.
If you are traveling with something very valuable, it's best to use the hotel's safe deposit boxes. I don't know if WDW has these, as I have never gone looking for them, but the safe deposit boxes are typically located in a locked room behind the front desk guarded by security cameras usually both inside and outside. You need the manager's master key as well as the key they give you to open it. Without one or the other, it cannot be opened. If you lose your key, they'd have to call in a locksmith or the company who owns the boxes. This is the safest way to store items when traveling, if you're concerned.