This is what i would do. First, use very small stitches, they don't effect bowing, they stop your quilt from coming undone. I dont usually sew in long rows, because it is very easy to get bowing that way. I usually take 3 or 4 squares from 3 or 4 rows, and sew them together like a 9 patch or 16 patch, then press, then lay on grid and square off any imperfections that have begun. Then take all these big patch blocks, and sew them together in rows. What happens is no matter how hard you try to cut and sew perfect, there is always a little imperfection, and a little3 imperfection in one seam is not much, but when it is multiplied by 3 or 4 seams, it gets worse and worse. Now, because i have never done a bargello, i dont know if you can do it like i suggested....sooo....this is the other option i would do. After sewing each row, do a very light pressing, just up and down with iron. Then lay it out on a grid, and see if there is any bowing. If anything is too small, you can then stretch it a bit with the iron, if there is any bowing, you can then trim it with your rotary cutter and ruler. If you do this with each row, then it will fix any imperfections on each row. I'm sorry about the rows you've already done, the parts that are bowing out, can you re-stitch your seam, and just make your seam slightly larger in the areas you need to, to fix it??? Instead of ripping it out?