If there are only scraps/strips of the backing fabric left I put it in my scrap bin for oversized pieces. If less, then they go into whatever bin is the closest to the smallest width. Either way it gets used up in another quilt. Even the cutoffs of batting if wide enough will get attached to other strips of leftover batting until it becomes large enough to be used in another project. Sometimes I'll use the leftover batting if large enough for embroidered quilt blocks. Also those really small pieces I use to clean my rails on my quilt frame using alcohol.