[MrB]

Don is right - there are no real rules - although the original quote was probably "There are no rules for good photographs, there are only good photographs." - Ansel Adams. Do whatever you want (or sometimes need) to do to get the final image as you want it to be.

Sometimes cropping might be the only way to actually capture the subject, e.g. when the focal length(s) of your available lens(es) cannot get in close enough, and there is some physical obstruction that prevents you moving any nearer.