If the spring tention was meant to be releived, it would state so in maintenance literature or original catalogs and care instructions printed by the manufacturer.
I believe most guns were meant to be left as is when done firing them.
Not always. When I visited the Darne factory in St. Etienne, the gunmakers always snapped the triggers to dry fire the guns. I asked why they didn't tell folks to do that in the directions that came with the guns. They made the point that dry firing the gun, without the barrels attached, placed extreme pressure on the faceplate, and would lead to bent retaining screws and the faceplate projecting out of the sliding breech. They figured they couldn't personally instruct every owner on the "dos" and "do nots" of every Darne in existence, and bent face plate screws could interfere with safe operation and lockup, while broken coil springs usually didn't cause any problems at all on a Darne, so no mention of it being safe to dry fire a Darne was ever put in a catalog, or, the instructions.
When you handled a Darne at the factory, and closed it, and pointed it in a safe direction and snapped the triggers, you would get a smile and a gentle nod of the head from the gunmakers-you knew the secret handshake, so to speak.
It is OK to dry fire a Darne, with no snap caps needed. Just make Damn sure the barrels are in place and the action is closed on the barrels when you do it.
There is also a method one can use to actuate Darne ejectors, without firing the gun, to use the ejectors to remove unfired rounds from the chamber. The factory, back in the day, had information on just how to do it, but, discontinued the practice of showing people how, pre-war. It is dangerous, again, if you don't know exactly what you are doing.
I don't show anyone how to do it, either.
Best,
Ted