I recently acquired a Finnish captured Mosin 91/30 Tula 1939. this would be my 3rd Mosin Nagant (izzy M44 1944 and izzy 1942 91/30).  

I have always stored them "decocked" and bolt closed, is this the proper way? Or am I doing it wrong?

I just want to make sure I'm not damaging them is all.

Probably won't make any difference how you store them. I'm thinking that when the Russians stored them for over 60 years, the bolt was closed and probably left cocked. 

In a crate with 19 others! 

That whole spring wear thing is a gun myth. Springs get worn out from however many cycles of being compressed and decompressed, not from sitting still at either position. So it doesn't matter.

Rust is the thing to worry about. So either leave the cosmoline on them, or today I'd use RIG (Rust Inhibiting Grease).
Ideally, they should be stored in a climate-controlled space, or at least with a dehumidifier. If fresh air is not circulating around them, then the air trapped next to them should not contain more than 40% relative humidity. If sealing in an airtight container, place sufficient dry desiccant (to draw all moisture in the trapped air to it) away from the metal so it cannot contact the desiccant. Keep in mind the contents may shift if moved, and secure everything accordingly.

I keep them de cocked , I don't think it matters much .