My advice would be to shoot a roll of film of the same subject at all shutter speeds and adjust your aperture to compensate; if there are any major differences in speeds, this will show up when you process the negatives.

Personally, I've never heard of this problem specifically with FM2's and have been using them for years. However, as John Koehrer mentions, any camera model/manufacturer's shutter may produce a speed with a slight variation on what it's supposed to be firing at, but it won't be significant enough to produce over or under exposure.