I think a square photograph in a recangular frame looks much nicer, at least for the sizer I work in (approx 7x7 image in 11x14 frame). I mount the image equidistant from the sides and the top, so it's square to the top, with a large space underneath on the bottom. Perhaps it's just the size I work in, which is a bit small for medium format, but I have never heard anyone, including my many photography professors who certainly know how to critique, say that they thought my framing looked funny. I have had a number of viewers tell me that my framing was very nice.
However, if I were going to print very large then I would almost definetely use a square; I think my preferred method of framing might look a bit odd with a large rectangular frame.