I generally use a bit longer than default for piecing, but if the pieces are small or the stitching will be cut (like in a bargello) I will use a smaller stitch. If your stitches are coming loose, it sounds like you are pulling against the seam. I never backstitch, but I too have had that issue, especially when the quilt top gets bigger and heavier. I have fixed that by stitching around the outside edge (1/8") catching all the seams. It won't show when the quilt is done and gives the piecing stability.