When I sew onto paper I leave it on until the blocks are all sewed together. Like someone already said, if you use scraps they're not always on grain and have a tendency to stretch. If left together until all blocks/strips are sewn together you have less chance of getting wonky blocks. I use an orange stick or one end of my purple thang to run down seams before tearing off paper. When you use a shorter stitch the paper is like perforated paper. If little bits remain, I just leave 'em. They can't be seen after sandwiched and quilted.