I think everything I've read about storing guns, including my owners manuals recommend storing them unloaded. I think we can all agree that an unloaded gun is less likely to go off and hurt anyone.
One nit to pick: Guns don't just "go off". The phrase "don't worry, it isn't loaded" is often followed by "it just went off!", usually because some put their booger hook on the bang switch.
As for owners manuals, I had a Taurus manual years ago that stated something to the effect of:
"WARNING: Storing a loaded firearm may result in serious injury or death."
A few "warnings" later, is stated:
"WARNING: Storing an unloaded firearm may result in serious injury or death."
Either they have a sense of humor or they were trying to cover their butts from every angle.