Sheesh, I always end up cropping images, even tiny amounts... "aw, didn't notice that blade of grass in the pinhole shot foreground". I rarely matte or frame anything right to the film format size. And I like that, there's composing while shooting, and then there's printing and re-interpreting, and often a bit of cropping makes the composition stronger. I couldn't imagine constraining output to some set proportion or paper size. My final proportions are what the image itself seems to want to be. Mattes are cheap.