I think the two most important things are placement and value. If you get your stuff in the right spot and the right shade, well, then you've got yourself an image that gets the point across, even if the shading is a bit iffy. I prefer chicken scratch images to really smooth shading, I must confess.

A project I've seen done before is a simple colored picture of a red ball or green cube, then trying to render the values in pencil exactly. After ya finish, just take the picture and turn it to greyscale and then compare that image to your finished one to see where the values and shading are a bit wonky. Really helps!