Of course my external hammer guns are uncocked unless ready to fire, but I think the issue centers more around internal hammer guns that automatically cock by either having been just fired or by cycling unfired rounds out before storage. I have read about using snap caps, and I have read heated arguments in other forums about whether or not this is necessary or helpful.
So, here's my take. Neither myself, nor anybody in my family uses or has used snap caps, so therefore, all of our guns are stored cocked. As for the effects on the gun, the oldest gun we still use every hunting season is a Rem pre-M11 (1930's vintage), and it shows zero effects of wear from being stored cocked for 80 years. My personal firearms subject to being stored cocked:
1958 Browning A5 (auto)
1950's Spanish Double (internal hammer)
1960's Ithaca 51 (auto)
1980's Weatherby (pump)
All these guns are flawless year after year.