In my mind, the subjects of my photographs are what should determine the "shape" of the final result.
The aspect ratio offered by the camera is just something that I need to work with in order to waste as little real estate as possible in the resulting negative or slide.
Of course for slides, it may be more important, because they are often projected (by me).
It probably helps that I both do my own darkroom work, and cut my own mats.
By the way, I shot a fair number of weddings where I was mentally cropping every 6x6 shot I took, as I took them, because 4x5 aspect ratio usually works best in most wedding albums.