Occasionally, I'll be out with the 4x5, but come across an image that should be square, so I'll crop in the darkroom. My basic rule is to try to use one entire dimension (i.e. the full 4 inch, or 5 inch length) of the negative. I want to utilize as much real estate available on the negative, but the final image is what's important. For those that don't crop, I'd ask how many shots you've passed up because you don't have the lens/format/point of view required for a particular image?