In the long run it won't matter. When all your blocks are done, measure them then square them up to the smallest size. Don't stress about it, it's your creation so do it your way. However, most of us sew a "scant" 1/4" seam which is about two or three threads LESS than a 1/4". This might help in your measurement. I, too, have had some that didn't quite measure up, especially when sewing on the diagonal like with half square triangles. I just trimmed the blocks down a little smaller and added a very narrow border to them. Unless you're wanting to enter contests and have them judged, just get them done and you'll be much happier.