I'm pretty skeptical about the Earth's magnetism being a real factor. I suppose it could be true, I've never researched it, but that has the sound of urban legend. I'd be much more concerned about local fields (putting disks too close to speakers, a TV... magnets hehe), climate issues (excess humidity or dryness or whatever is worse, heat, etc), physical damage (dirt, bending) and in the case of C64 disks, banging 1541 heads reacting to lame copy protection or bad alignment.
Of my disks, my Amiga disks are fairly new. My C64/128 disks go back to like 1984 and most disks are from 1984-1992. I have a few new disks Ive bought in the last 2 years. Of the old disks, about 10-15% are bad. But most of those have BEEN bad since the 80s... dud disks, alignment manglings, physical damag, stuff like that. Of the disks that worked fine in the 80s and early 90s, most work fine still. 1% seems a bit too low for my results, but its probably under 5%. Considering how many of these disks are over 20 years old and how many times theyve been used, reformatted, given new data, etc, I'd say thats damn impressive.
