When I crop, it's usually an artistic decision. It has nothing to do with keeping the "good" portion of a photo (okay, sometimes it does). Sometimes what I choose artistically at the time of shooting is different than what I decide when it comes time to make a print. Some images look better with an odd aspect ratio, some don't. I think it's probably all personal preference depending on the photographer. Cropping is just another tool to help express what I want to.

-Taltos