I start with my squares that extra 1/8" larger (i.e. if size required is 3 7/8" I cut 4"). To add to what PaperPrincess said - when squaring up and/or trimming your HST's, measure from your seam - not the total square size. Measure 1/2 the size from the center seam to the outside edge and then trim; turn block around and do the same thing on the other side. This should give you a correct size block with your center seam exactly on the diagonal. Then, when you sew to the next unit, you should not lose your points if you are sewing with a good 1/4" seam.