Petercat, your observation is a good one. With a non-orthogonal move, the width of the layer would be something other than the dimension of the square's width. That's the best reason yet for a cylindrical nozzle opening. A circle is going to have the same diameter in all directions and the layer will be the same width.

Imagine the assembly instructions for a hot end with a square opening! "Examine the nozzle with a microscope, ensuring that the edge is parallel to the heater block. When placing the heater block, ensure that the edge is parallel to the bed." Yikes.