I always crop my 6x6 to a rectangle, mainly because I am cheap - I mean, on a 8x10 sheet of paper, I paid for that 2x8 rectangle I am not using!!!
Seriously though, being able to compose a picture exactly the way it should look on a negative is quite a skill, one that I am striving for, but am a long way from achieving. Also, the type of shooting I usually do forces me to frame as best I can and hope that "my picture" is in there somewhere (and this may be an excuse for lack of above mentioned skill...).
I don't think (and this is my very humble opinion) that cropping is a bad thing, thoguh. Sometimes, unless you are shooting from a tripod, looking at a posed or better yet, immobile subject, you just don't have the time to take all possibilities into account, and you do the best you can. In the dark room, with the image "captive" on your negative, you have a lot more time to examine every angle, all possibilities, etc. - so you crop.
Yesterday, I realized that I have been conditioned to "fill the paper I have". I had a shot of a Hawker SeaFury (fighter plane - late 1940's), taken head on from ground level. This was a carrier based plane, and had folding wings. Its wings were folded up when displayed. I found the shape of it fascinating, so I took a shot that I carefully composed, and succeeded in getting onto the negative exactly the way I saw it in the (square) viewfinder.
Well, I think that poor easel of mine was ready to call Dr kavorkian - I turned it this way and that, adjusted it, zoomed in, out, etc. - until I realized...hey, I actually got this right! I am not sure where that conditoning comes from - I wold really like to think that my opening statement was purely humorous.. but perhaps it rings more true than I thought! Or is it schooled into me? Now I have a beautiful silhouette of a Hawker SeaFury with wings folded, only the huge spinner and prop showing detail and lovely, wet-looking highlights... printed on a perfect square! I am proud of that white strip on the bottom - it is a testimony to the shackles I threw off!
But in the end, if I was to show any of you a picture, square or rectangular or circular - whatever the shape, that you absolutely loved, you thought it was God's gift to photography, would you change your opinion if I told you afterwards it was cropped?