I don’t change out springs. The cost difference isn’t enough to mess with them. Just sell the old traps for cheap and buy new ones. If someone has a bunch of free time maybe it’s worth their while...

I have left some compressed and some not for many years. Never noticed a difference.
.

From Boco:

Of course not. There's a rational limit to storing and taking care of things. Fully removing the spring would be excessive. But throwing them out on the ground and running through them with my setters is easy, so I take a bit of time to do it.

From Boco:

The amount of tension does matter though. The greater the tension, the more creep that will occur. There's a big difference between compressed and uncompressed (but still on the trap).

You may be confusing creep and set. If I remember correctly from my college days, creep was a pretty high temperature phenomenon in spring steel. Set would be what causes an initial weakness from compression but only initially. It was a while ago for me though.

It's my understanding that creep is often associated to prolonged exposure at high temps, but can be caused by any stress for a long period of time, even at low temps.