Its extra work, but when it is all sew together it is worth it. Press each block as you finish it, square it, then sew into a row, press the seams, then square the row. When all the rows are completed, measure vertically in three separate areas. Add the measurements and average. This will be the size of side borders. Measure horizontally, including the borders which you have sewn on. After is is quilted, check for squareness. Probably very minimal.