I generally try to compose in camera as much as possible, but I'm not adverse to cropping to improve the image afterwards (sometimes even going a bit far for my 6MP camera, to remove distractions etc.)
With action shots I've frequently found I don't leave enough cropping room, & get photos with the edges of the main subject just out of view, ruining the shot. Hopefully I'll learn to leave a bit more for subsequent cropping.
I generally prefer to keep the aspect ratio, it was chosen as a pleasing ratio to the eye after all, but I've had several images where this didn't work at all & I've changed the ratio drastically, going to 3:1 or more.
I sometimes think that every rule in photography needs to be broken for one image or another - The only exception being 'if you like it, it's good'.
