On reflection there are many reasons why I favour a square format when creating directly downward photographs. For starters, when I first came across Landsat Satellite Images they were invariably square. The Boyle Family, who were an early influence, made square artworks that were replications of the surface of the earth.
I also remember that the very first two screenprints I produced on my Art Foundation course were square in shape.
More significantly, when I produced screen prints for my first proper exhibition, the square format allowed me to work more intuitively. I could print new layers of ink from any of the four sides through turning the image 0°, 90°, 180° or 270°.
Lastly, rectangular images are usually described as portrait or landscape format, depending on if they are vertically or horizontally aligned. Neither of these descriptions seem appropriate for a downward photograph where any of the four sides could be regarded as the top. A square format allows the image to be legitimately viewed from any of four positions without it changing the essential balance of the composition. Naturally, through the editing process, I choose the orientation I prefer, but if anyone coming across my work favoured the image in any of the other three positions, I would have no problem accepting their choice.