In my youth, you could comfortably rip fabric on the WOF and have a straight edge. Now, not so much. A lot of the fabric is woven "funny". And when I have seen someone rip fabric recently, you have at least 1/4" to 1/2" that is not usable due to stretching and raveling. I have never tried to rip down the selvedge. I didn't even know that people did that. Your long armer is not going to want any extra ravels or stretchiness. Consult her before you do anything with the backing.