If I can, I try to stagger the seam allowances, both for the sake of the weight and because the seams line up really well if you can push those s.a.s against each other. (They sort of lock.) But if I get done sewing a seam, and find out some came out pointing in the wrong direction, I usually just press flat and let it go. Once in a blue moon I might cut the seam open with a blade and then re-sew with the s.a. lying in the other direction. I guess it depends on my mood and how much time I have.