I have spoken to no fewer than three Mechanical Engineers on similar issues related to springs that have been brought up in discussion forums, such as:
"Should I store my spare pistol magazines loaded or unloaded?"
"Should I store my cameras with shutter cocked or uncocked?"
Every time, I was told that having the spring compressed over long periods of time does not weaken it. What weakens springs are compression/decompression cycles.
That said, I leave my cameras uncocked except for the Hasselblads. In that case, it reduces the chance of me removing a lens from an uncocked body (besides, the motorized Hassies are always cocked).
For auto lenses, I leave them wide open; for manual lenses, I do open them to maximum aperture. Somehow I think this helps keeps the blades lubricated. However, I confess that I truly don't know if this is the case or whether it helps or not.