...
I think storing the gun assembled with snap caps in and triggers pulled is probably "best", if dropping the hammers to relax the springs is your intent. For this purpose I would advise in favor of plastic snap caps (and definitely not old shells as snap caps) so as to avoid even the possibility of a reaction between aluminum caps and steel barrels (aluminum and steel will set up an electrolytic reaction if there's any moisture) or, worse, steel on steel possibly rusting together if there's any moisture.
I place a hardwood dowel down the bore and press it up against the breech,,and dry fire the shotgun to let the hammer spring tension off.
As large of diameter that will fit, and face off the end nice and square. Simple, cheap hardwood dowels from the home improvement store will do. They're usually birch.
A strike onto the wood will take place somewhere on the end, not always in the exact center, so it can be reused many times before facing it off again. Plus you do have the other end to use!
No snap caps left in the chambers to think about.
I usually can't find the things anyway.
Reality is as 2-piper says, you can't write a simple single set of directions that is applicable to all makes & models of double guns.
What ever works best for you and doesn't damage the gun in disassembly or re-assembly.