Do you realize that you can not put the AR-15 selector switch to "SAFE" unless the charging handle has been cycled, placing the hammer in the cocked position?
My co-worker is a USMC Reserve Marksmanship coach, and he states that their weapons are kept on the rack in the armory with the the hammer released, the bolt forward and the selector switch in the semi-auto position.
When the weapon is issued, the armeror cycles the charging handle, verifies that the chamber is empty and hands the weapon but first to the marine with the bolt locked back (and the hammer cocked). The marine then double checks the chamber, smacks the bolt release, closes the dust cover and places the selector switch on safe.
As long as the weapon is verified empty, I don't think it matters whether the safety is on or not.
I don't put mine in the safe with the hammer in the cocked position.... hence the safety can not be on. I understand that it does not adversely affect the hammer spring to store it with the hammer cocked.... but have always made it a point to store any tool at zero potential energy. The exception, of course, being a weapon that is intentionally strored in a ready condition (i.e. carry gun in the bed side gun vault)