The sancrosanct rule against cropping is the instrumentalization of a particular theoretical positioning: if making a photograph involves framing and selecting in a manner representative or expressive of one's vision, then the value of a photograph should be proportional to the value of the visualization effort done at the moment of exposure.
In other words, if you believe that the original act of seeing is of paramount importance in the final result, then you will maximize the means to demonstrate your talent, i.e. by not cropping.
This kind of aesthetic principle aims at being a generalizable one, and therein lies the problem for me. While I agree that a good eye needs training, and that trying to match the camera's point of view with one's eye (by shooting full-frame) is a great exercise in compositional rigour, I don't think that's the royal way to photographic quality. Far from it. By doing so, you are cultivating only one kind of seeing.
My habit is to try to see as best as I can when I shoot, but whatever needs adjusting in the darkroom gets done. I have a tendency to prefer accomplishing a full-frame picture when I shoot 6x6 square rather than 24x36 rectangle. I'm a lazy rectangular: most of the time images from my 24x36 negs will end up being whatever fits nicely into a 8x10 or a 11x14 paper. Obviously, there are also pictures that I always print full-frame, since that's how I like them best anyway. In other words, my rationale for cropping or not cropping is often one of convenience and process.
But I urge anyone to disregard the teaching of "Masters" (tm) who want to tell you what "Good" (tm) photography is all about. HCB is not God, and will never be. But those people running workshop sometimes behave as if they are...