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It would appear tappet or valve springs in most engines don't mind being at full compression for long periods of time either or we would also end up with more broken/failed springs. When the engine is shut off, at least one of the springs is going to be at or near full compression often and maybe left that way for some time.
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Just because a tappet spring is pressed down as far as the push rods and rocker arms can push it doesn't mean it was fully compressed or that it has reached or gone beyond it's design limits.
Similarly, just because a magazine is fully loaded or a slide is locked back doesn't automatically mean that either spring is fully compressed.
In the case of some compact or hi-capacity mags, fully loaded/locked back clearly pushes those springs to or beyond their limits. In the case of some recoil springs in full-size gun, they can't be compressed much farther. But that isn't the case with every gun, mag, or recoil spring.
Fully compressed means just that -- it's a specific state in which the spring can't be bent or pressed down farther. Engineers familiar with metals and spring design tell us that springs that are FULLY compressed (or at or near their design limits) and kept in that state for longer periods, will not live as long as springs that aren't worked as hard. Go to the Wolff Spring website and check their FAQ area and see what they recommend for hi-cap mags, for example.
Talk to someone who works with spring-powered air guns -- and there are a few who participate here. They'll tell you that leaving such a gun fully cocked leads to relatively rapid degradation of the springs that send the projectiles down the barrel.
Better yet, read some of the technical info in the links above, or do that search on "mag springs" I mentioned originally -- you'll find a wealth of information here, some provided by engineers who have first-hand experience with the topic.