Depends on the final use. If it's for the Web, I'll usually crop to 800px on the longest edge so it's suitable for most sites I'm likely to post it on. I usually host on Flickr at that size to avoid their resizing which tends to over sharpen them. If I intend to print, I'll generally crop to the A series dimensions, although I always keep an original so I can revisit if I want a different ratio.