As my mentor taught me many years ago..."When you strip away everything that is unessential in a representation, what you have left is art."
This applies to all fields of art, which, of course, includes photography and in photography, cropping is the simplest and easiest form of stripping away the unessential and when done properly, what you have left is art.
The best time to crop is when you take the shot. The BIGGEST mistake, by far, that beginning photographers make is to include too much extraneous detail in their shots, which results in what are commonly called ‘snapshots’. While composing a shot, isolate and shoot only what it was that caught your eye in the first place.
Following that is often further cropping in post-processing, to correct for errors or for what couldn’t be done in the camera.
The series of images below illustrates compensating for a composing mistake, as well as removing distractions during post-processing that couldn’t have been removed in the original scene.
Click on the thumbnails for further information on each image.