It's an "almost" dogma. I find it a good practice to work within the frame I am given. I like the square a lot, and if I am shooting with a Mamiya 6, why would I want to start fiddling around with verticals and horizontals? I like the 35 mm ratio, and 4x5, although I am not crazy about 5x7. It's not that HC-B didn't crop (although he did crop what was probably his most famous photo, In front of the Gare St. Lazare.) Walker Evans cropped quite a lot, and there are no real rules, but I like to work with my first vision, as well as what is happening at the edge of the picture. I also shoot either in B/W or colour, even in digital, and believe you to figure out what you are doing when you are shooting. Of course, digital workflow renders everything moot -- you can do so much with an image, but there so much stuff that looks to me artificial and over-processed. I have been out this evening with a new Leica Monochrom. It is liberating to be able to shoot in really low light.