I've preferred the square format for years. One big advantage is when shooting on a tripod, you've no worries about having to revolve the camera awkwardly on its side to get an upright format shot. (only the Mamiya RB67 series had "revolving backs" to counter this, and they weigh a ton!) Basically you frame the picture till it "looks right" in the viewfinder, then if you want a vertical or horizontal shot out of it you merely crop in a bit at the enlarging stage. However the square shape itself shouldn't be dismissed, many pictures look best that way. I've had pictures published as a square shape.
If you are definite that a rectangular shape is required, then leave a bit of "give" around the borders. Many viewfinders will have rectangular lines near the edge both horizontally and vertically to guide you.