I crop in two stages. When I am developing a picture I crop the image, mostly landscapes but it can happen with animals. A good way to crop your image is to get a large mount or mat and cut it in two diagonally. You can bring these two together, adjusting how big you want the picture to be. It’s a very good way to see what a picture could look like and gives you an idea before actually starting the drawing.
When I’m drawing a picture, I make it a little bit bigger so that it can be “cropped” a little when mounting. Cropping becomes useful when it comes to signing pictures. I can find the right composition of the picture before it is signed and then the signature is in the perfect place. A good idea is to sign a picture after you mount it to ensure the picture is right.