When I got my first TLR, a Seagull, I marked the focussing screen with crop lines for the horizontal and vertical 8x10 aspect ratio.
After a few rolls of film I noticed that the pictures were coming out square. I mean I was ignoring the crop lines and just going with the native format of the camera. I guess the psychology was unless the image looked good I didn't shoot and looking good meant fitting the square!
It works the other way too.
When you don't have to turn the camera to switch between a horizontal crop and a vertical crop, you end up cropping mentally, both in the viewfinder, and in real life, without a viewfinder.
Cropping guides on the screen or the frame around it can be useful, if you are trying to photograph something in particular (like a bride and groom for an album photo).
If, instead, you are looking for a special photo, the constraints of the screen, or the paper may matter less.
For what it is worth, if you print a lot, on to 8x10 paper (or something similar), you tend to get really instinctive about formatting in camera for the resulting crop.
Matt