If the old seam is in the new seam allowance and I don't lose points or such, it stays in. Unless, it is a seam where a bunch are meeting and the bulk is too much. It's pretty much a personal choice. Now, if I have fabric that is tending to fray too much, I have been known to sew a wavy seam within the seam allowance to try to stabilize the fabric. And it works for me.