On a stitch and flip block (snowball), I sew a little to the right (1 thread) and that seems to help. I also only trim the center piece out - that way if it doesn't turn out perfect, I can still use the back piece (which is cut to the right size) for lining up the pieces.
When just making HST, I always oversize the cut and trim after sewing. It is much more accurate this way.