I would agree w/ the consensus here. Make it any way you wish. But for some strange, bizarre reason, whenever I see a rectangular format pic that I like and later discover that it came from a square format camera, something in me rebels and is disappointed. I have no idea why. This also happens when it's a 35mm format that has been cropped to square format.
Whenever I compose a shot w/ a TLR, 4x5, or 35mm camera, that's how I print it (for the same compositional reasons). But, that's just me. If I don't know that a pic has been cropped, then it doesn't matter. It would probably take years of therapy to understand why, but the why is unimportant. It's just the way it is. At least I don't file the neg carriers out to prove that. They sometimes get filed because they're often a little too small, and cropping my image on their own.