Enlarging lenses are flat field and one is dealing with a minimum depth of focus, not to be confused with depth of field.

There is a reason we focus on grain, if one considered that there is a good deal of tolerance then the need for grain focusers would not be needed
as we could slap a negative in , focus by eye at a distance and close the lens down.

I have never found this to be true, I have my enlargers bolted top and bottom and laser align constantly , I also use glass carrier and Apo lenses slightly longer than normal.

I rarely would dare disagree with Ian Grant who is my chemistry Guru, but in this case I will.