I always compose for square with my Hasselblad and rectangle for my 4x5 and 8x10 cameras. The only time I crop an image is with my digital camera like I used to with 35mm cameras. I have never cared much for the longer 35mm aspect. There is nothing wrong with it, I just don't "see" that way.
To me, composing square is a creative break from composing the standard rectangle which I also enjoy.