It's probably not that your seam allowance is inaccurate, but that the fabric that folds over the seam takes up a little bit of extra fabric. So your scant seam allowance might not be scant enough. There have been times, when I haven't been very accurate, that I've needed a 1/8" seam. When that happens, the seam can fray/tear out easily. So I run two lines of stitching on the seam allowance very close together. It can be a little more difficult to hand quilt through, but the seam won't come loose.