I've made many rag quilts. To stitch an x or across the corners to prevent shifting of the back, front and any batting or additional fabric layers depends on the size of the block. Larger blocks should be stitched in some way in my opinion. Smaller blocks could just be tied in the center. 6 strand embroidery floss, knitting yarn and 2 strands of crochet thread have all worked for me for the ties.