I know Bob Carnie disagrees but actually it's not as critical in terms of sharpness as you might think if a lens is stopped down. I occasionally use tilt of the head and lens board with my enlargers to correct perspective. However with an older 5x4 I used in the past this wasn't possible so you needed to tilt the enlarger easel, even with a 1" or so difference one end to the other on a 16x12" print there was no loss of sharpness stopped down around 3 stops.

What's more critical is parralell alignments to prevent distortion.

Ian