OMG! DON'T undo do it and don't give it any more thought! With white batting, you'll never even notice it. The only thing I would check, is to be sure there's not a sliver of darker fabric that is a bit bigger than the white in the seam allowance and might show through. Then just carefully trim it off. Another don't - don't point this to everyone who admires your quilt when it's done! Pressing to the light is a GUIDELINE, not a rule, as many times the fabric needs to be pressed to the light side for construction reasons.