Ignore my post of 07-08 at 4:30 PM. The test I described is to detect whether the weigh pan of a digital VTF scale is ferrous (some are); it obviously won't work to detect a magnetic attraction between the cartridge magnets and a ferrous platter or subplatter.
psf, After stating that you have no issue with the TD124 platter, you go on to describe what I would call an "issue". Anyway, it sounds like you know how to deal with it satisfactorily. I don't know about the strength of the magnets used in Grado cartridges, but I do know that my Ortofon MC2000, which seems to have its magnets in the "belly" section of the cartridge body, hanging close to the LP surface, definitely does have issues if you try to use it with even a stainless steel platter that has significant iron content.