Interesting discussion. I shoot mostly in a 6x9 format (35mm or 645) or 6x6. I mostly print full frame unless I screwed up and ended up with something distracting in the corner of the frame. When I first started, though, I was the one saying that the format shouldn't dictate the print proportion. What changed? In my case, I think I simply improved my compositional skills and I could better utilize the full frame....or it was laziness as I started matting and framing more...(: I used to shoot 4x5, but I never got the point with that format where I tended to use the entire frame and I still tend to print 4x5 either square or longer and narrower. I think I simply never became skilled in composing in that aspect ratio. I'd be curious to know if I am the only one who followed that progression or whether I am an anomaly.
Oh, and do get a 4 bladed easel if you want to print square. It makes life easier and allows for wider borders.