As an engineer myself, I agree with your engineer friend. As a restorer of double guns and maker of springs, I can add this. I see many guns that have been sitting unused
and left cocked for decades. When I remove the parts, clean and then reassemble, the springs are just as good as when they were made over 100 years ago in most cases.
Also consider this, most people at the turn of the century didn't use their guns any where near as often as we do today, with clay shooting and all, I would wager most were all kept in the
cocked position, when put away.