I never double stitch seams, just use a shorter stitch. I do back stitch when I am piecing together a block and the start/end of the seam will be on the edge of the block. I've found that the very ends of the seam will separate a little until they are secured by the next block, sashing, or binding. I prevent this separating by doing a 2 stitch back stitch. It isn't a big deal if I have to rip the seam out. It also saves time and frustration putting blocks together when the very edge of the block is still together, especially whe dealing with triangluar pieces cut on the bias.