I don't know. My first medium format camera was square format. I've tried to stay with the square format to simplify things. I crop sometimes, but I don't really think about it. I just consider the image area available to me on the ground glass and use what's there. If there's a distracting element, or something that sticks out, I just crop it later.
It doesn't seem that important to me.
"Often moments come looking for us". - Robert Frank
"Make good art!" - Neil Gaiman
"...the heart and mind are the true lens of the camera". - Yousuf Karsh
I learned to photograph with a Rolleiflex -- I love the square. Instead of a linear side-to-side (or vertical) eye movement across the image, the square encourages a circular eye movement. Wonderful to work with. But then I also like working with 4"x10" also.
At least with LF landscape, a bad day of photography can still be a good day of exercise.
I think the best advice I've received so far was Les McLean telling me to not be constrained by my viewfinder. I do shoot square in the landscape (although not for true vista type landscapes) and generally print square too, but since paying heed to Les' advice, I have opened up a new world of opportunities from my 6x6 negs - crop, crop, crop!
I am with Leon. The square provides the opportunity to use any part of the image vertical or horizontal when printing. I seem to use either a square format camera or a panaromic. The common 4x5, 8x10 rectangle is boring to me.
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I think that since we have binocular vision and our natural view is somewhat rectangular, it's natural to go rectangular for landscapes. For that reason, I bought a 645 camera...but I use my 6x6 Rolleicord more. Go figger.
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Like so many others here, I have shot 6x6 since I got one of my first cameras back in 1960 and I print 95% of everything I shoot square as square... I feel more comfortable seeing that way from 40+ years of doing it... and when I have a boring 4x5 image, I often find that there is a great square image hiding in it.
When I do crop one of my 6x6 landscapes, I find that the most often crop is to a 3x6 image (2:1 L:W) which is a great landscape format for the right image. I think that a 6x12 back for the 4x5 would be a great addition to the kit, for that reason (although I know that I can crop a 6x12 image out of a 4x5 negative...).
But the fact is, I would still mostly shoot and print square.