First, all springs compress under load, and never _quite_ return to their original shape, until they have "set". The "set" on a well made, normal spring allows it to meet the desired spec, and go on at that level through a long service life. The "set" on a bad spring (rare, but they are out there) is usually noticeable as an out of normal coil, or a dead section when under compression.
All that having been said, you can prolong spring life by downloading one round. You can safely load them full and keep them full for long periods of time. The savings in spring life by downloading and the "stress" of keeping them fully loaded are negligible. Chances are your kids _might_ see one of your springs fail long after you are gone.
Springs are like anything else mechanical - the more you use them, the faster they wear, and wear on a mag spring is loading and unloading, and resulting rub wear against the mag walls. They are designed for this. Shoot. Have fun. Worry about real stuff
