I know it's an old thread but feel I can put this out there to help those who haven't done this. I don't have access to a shop and the "right" tools so I proceeded to look for any ideas on ring filing by hand and maintaining a square edge.
To add to the thread: I don't see how clamping the file in a vise and cutting both ends at the same time would keep the ends squared to each other. The thickness of the file would prevent this. May be "acceptable" if the file was only ~.020 thick. When you put the ring in the bore the end at the cyl wall will be tighter than towards the piston. ....Unless you're running a gap the thickness of the file. which I doubt.