I have no problem with cropping in the darkroom, although I will admit to an irrational sense of satisfaction when the best choice for a negative is a full frame print.
When I expose the film, I will generally try to fill the frame with my subject, but if faced with a subject that appears strong in a number of different ways, I'll try shooting them in all those ways, and/or shoot with enough room to permit making a cropping choice later.
I think it important to remember that when we look through a viewfinder or at a ground glass, the image we perceive is affected by the frame around it that the camera provides. That frame may or may not affect us in the same way as the frame provided by the choice we make in respect of the final presentation of the print or slide. As an example, a subject that appears one way when viewed through the viewfinder's "tunnel" may appear different in a matted print.
I expect as well that at least part of my desire to compose so as to avoid having to crop comes from early experience trying to print 35mm negatives with obvious grain and mediocre sharpness. If I had started with an RB67 and properly exposed, perfectly developed low speed negatives, I would probably be sloppier than I am now .