>I'll usually let the "picture" dictate the final >crop of course.
Well this is said a lot. The funny thing is that if you go to an exhibit of a major photographer, you don't see images in different formats by being cropped in a variety of ways. You normally see very consistent image size and format displayed. If the picture truly dictates the format, would you see this consistency from major photographers? Just something to chew on. I only see inconsistent formats from the hobbyist who may display.
Okay...putting the artistic & creative thing aside, when someone is paying you to shoot images you need to be prepared to provide standard sizes.