Thomas, isnt a hearable difference an important difference?. 1 cm makes a big difference on 25 or 30 wt cones because the centre is so small. You can find a nasty phasey sound pretty close to the centre so ,...after all why eq it if you dont need to, all you are doing is messing with the phase.
You really shouldn't overlook it, 1cm back and forth from the grill gloth makes a difference as well, especially with the tightness of the low end
Left and right can sound different, depending on how cab is wired.
I always check every speaker as they all sound different, and if one is on its way out, it will alter the impedence.