I'm too lazy to look this up, but I once asked about "wearing out" springs on AR15 mags over at AR15.com. They pointed me to a website of "Common Fallacies" about AR15's.
One myth that was disputed on the website was that springs "take a set" after being loaded for a long period of time. The site stated that springs only become weaker after a full cycle of compression and decompression. In other words, it isn't the length of time that a spring is compressed that weakens the spring, but thenumber of times it is compressed and decompressed.
I have no clue how accurate this information is, but it certainly seemed trustworthy. Besides, it was on the Internet, it must be true. (*cough*)
If this is true, the best thing might be to repeatedly load and unload your mags. Does the HKS speedloader also unload mags like a LULA?
-James