Well, whenever I crop a photo later because I am not happy with my composition, I usually crop it so it stays with the print ratio, which - provided I were to reduce a pic to 800 pixels wide - is 800x533 for pics that come out of my 350D and 800x600 for pics from the Powershot. I keep checking until I get back there (always hoping I need not crop into anything "of value" that I'd actually want to keep).
In that case I can also have that crop printed if I wanted to.