The two most critical planes are the negative stage and the lens. If they are out of alignment, it is very difficult to make prints that are sharp corner to corner.

It is also important to have the lens and the easel aligned, but in the case of alignment failure there, depth of focus can help you a bit.

If the negative stage and the easel are not in alignment with each other, your prints will also be out of square - it is possible to have sharp corners, but skewed shapes.