To me the negative is all about capturing a moment, where the content itself dictates where I aim the camera. It's very rare that the content aligns itself with the aspect ratio of the capturing device; I just make sure I get what I want and worry about expressing myself at the printing stage.
For some reason I like the full 8x12" or 12x18" view when I print 35mm negs with landscape subject matter. Then, for some reason, I love the 3:4 aspect ratio for portraits, so that becomes 9x12" and 13.5x18" when I have portraits in the frame.
When I shoot 120 6x6 I usually end up printing a full square, because I like how that looks. Unfortunately I have a vision problem that means I'm terrible at aligning the camera so that it's parallel to the horizon or perpendicular to the center line, so even though I print a full square, often they are cropped due to negative rotation in the enlarger. (This rotation issue is even worse with 35mm where I often shoot hand held).
But once in a blue moon I'll crop the 6x6 negs to 645 ratio, because that is the 3:4 ratio I like for portraits. And once in a blue moon I crop 35mm negatives to square.
Go figure. The film format does not dictate how I shoot. Neither does the paper size. But to some extent the print area does. Call me crazy if you want, but it works for me. I don't think there's anything holy about the aspect ratio of the film, as I'd like to fully explore the potential of the negatives that I like. Whatever it takes to carry the content of the picture forward - that's basically the only hard rule I have.