I print square with the paper in portrait mode so that even just in a portfolio it has white space around it and more space below than above. Usually printing 6 3/4 to 7 1/4 square and thus the print has a finished look to it and it is actually easier to mat and frame as the window of the mat does not have to be as exact as I have the white of the paper showing.
I compose for the square as that is one of the major reasons I shoot with a Hasselblad or a Rolleichord instead of a 645 or 67. I also shoot 35mm and LF and in each case compose for the frame I have. Of course sometimes I compose with cropping in mind and sometimes I crop in the darkroom as I decide a different aspect improves the image but for the most part I try to be full frame. I do not think I am a slave to the aspect ratio of the camera rather I choose the camera for its aspect ratio whenever practical. I also just happen to love the square format of 6X6 and the more square ratio of the 4X5 over 5X7 or 35mm.
If you wish to crop to any other ratio or always print full page and let the paper decide, that is your choice. If however you do end up cropping a high percentage of your images to a more "standard" ratio in the landscape orientation than perhaps the 645 back or a 645 camera is more suited to you.