I don't start out with any pre-conceived notions of how I'm going to print things. When I use a sqare format camera, sometimes I see things that fit the square nicely, and I'll compose for that. Other times, I see a rectangle; the proportions of which may or may not fit the standard aspect ratios one finds in pre-cut papers. That's ok. I'm not against cropping, nor am I against printing full frame. It all depends on what you see and what looks best. In some respects, the square is perfect. It can go landscape, portrait, or square. That's what 4 bladed easels and paper cutters are for. But yeah, decide what you want first, and this is easily done with a china marker or a sharpie on a contact print - or even something as simple as two "L" shaped pieces of cardboard that you can use to mask off portions of the frame while looking for a good composition to print. Then trim your paper and use the offcuts for test patches or strips.
Bottom line? I don't let the shape of the frame determine the image. I just compose within that frame to get what I want to print..