Absolutely, positively ... let them rest. The same as you jack your car off the ground when you're not using it or disassemble the heads to let the valve springs, the springs of a magazine must be rested.
... or ...
No. As long as the spring is not worked past it's elastic limit, it can remain either compressed or extended for years upon years. If anything, it's the constant working, the compressions and extensions that weaken a spring.
I think this is just one of those myths that has just lingered on, perhaps because early springs were not exactly metallurgically advanced. Most were simple leaf springs, hand- or cottage-made.
This is just a forged and folded bit of metal
as is this:
I can easily imagine a stamped piece of sheet metal being used as a spring 'taking a set.' Aside from the 1911 do any other modern pistols make extensive use of flat metal springs?