For what it’s worth I use the mirror method as well, it is the only direct way of establishing alignment with the sensor, still it’s a debate that seems to endure.
I’m puzzled about the question of ‘scans’ being sharper when the emulsion is facing up, I don’t believe it frankly. Doesn’t seem to matter with my 6,300 ppi Imacon scanner and not with this 61MP Sony capture either: