If you believe that every good, photographicable scene is guaranteed to fit exactly into the aspect ratio of your camera, than you have no excuse for cropping.

And if you believe that it is always possible to stand in the location that is ideally suited for making a full frame photo of every good, photographicable scene, than you have no excuse for cropping.

On the other hand, if your real world includes square subjects, narrow vertical subjects, shallow horizontal subjects, round subjects, subjects suited to a 4/3 aspect ratio, subjects suited to a 5/7 ratio, subjects suited to a 4/5 ratio, subjects suited to a 2/3 ratio and subjects on the other, inaccessable side of the river, than cropping is permitted.

My rule of thumb? Get the most out of what you have in the camera, and then crop (where required) later.