I've owned more MLs than I care to admit and the majority of guns that come through my hands do not have a different POI from a "clean" bore (as described above) vs. a dirty bore.
That's been my experience also
TN. But the bore must be oil free.
I've found the first shot from a clean bore that still has oil in it is much more likely to be out of group. Whereas the clean bore shot from a bore that has been wiped with a very wet alcohol patch (followed by a dry patch or two) before loading is likely to be within group.