I shoot 6x6 and 35mm and will print "full frame" or crop the heck out of it. Sometimes square, sometimes not. Occasionally I'll even crop the top and bottom of a 35mm to make it long (or tall) and thin. I have two prints from the same 6x6 neg. One printed square and the other heavily cropped. Both work for me and suggest 2 almost different moods.
Square, rectangle, triangle, round, hey what ever works!
Oh and the last thing I do as I squeezing the shutter is sneeze.