There are a few things to consider:
The phenomenon known as creep, as mentioned above, only affects materials at or above ~0.4x their melting point, in absolute temperature (kelvin). This is unlikely to be an issue in regular service unless your springs are made of something absurd like lead, which actually creeps are room temperature. (I am assuming the temperature increases as a result of firing the weapon are small)
Stress-strain cycles, on the other hand, play a major role in spring wear. Ferrous material like iron and most steels exhibit an infinite lifetime under a particular amount of stress amplitude - not the absolute stress, which is generally far less - (the so-called "fatigue limit"). Less ductile materials like aluminum and titanium have a finite cycle life regardless of the stress amplitude; however, parts designed with these materials generally have lifetimes in the millions of cycles and fail by different modes long before the lifetime is reached.
So obviously, the life of the spring depends on proper design and materials choice. The spring steel that your gun would most likely use is a moderately-high carbon steel, with potentially nickel, silicon and manganese alloying agents in small quantities. Properly designed, it would last far longer than the other components of the gun that are regularly undergoing thermal stress, diffusion, and much larger fatigue cycles.
It's safe to say that storing your mag in a properly designed gun, will not wear out the spring prematurely. However, removing 1 or 2 rounds would increase the odds that you are maintaining the spring stress below the critical fatigue limit.
As a footnote, springs in regular circumstances follow Hooke's Law, which states that F=-kx (k being a materials, or "spring" constant and x being displacement). Thus, spring force is linear to displacement.
