The only bad thing I did was open the case with a magnetic-tipped screwdriver, which is the pissiest little magnet, and it only touched the case, so I don't think that would have affected it.
(I didn't know it was magnetic until afterwards...)
so, magnetic doesn't matter. I use them all the time. never had a problem. how else are you suppose to get those tiny littel damn screws from under the motherboard or pinned between stuff.