Originally Posted by Bob Carnie
I don't know that I would say that you are missing something...no doubt that perfect alignment is necessary for optimal sharpness.
However I have found that on the Durst that I can have perfect alignment while correcting keystoning or even correcting perspective as I did in one of my images. The distortion that originated at exposure was the reason that I didn't print one image for almost fifteen years.
I swung the lens and carrier stage and tilted the baseboard to arrive at perfect alignment and distortion correction at the same time. So long as everything is parallel, there are any number of points that can be defined as being in perfect alignment.
So long as the axis of the enlarging lens is in parallel orientation to the baseboard, there is no difference in the light level reaching the print.