Many of us have pieced "franken-battings". The quilting helps keep the fabrics/batting stable, I think a quilt as you go method would be pretty perfect for using up width- or length-wise pieces. Try and keep the loft/density similar.
Each of us needs to decide what we are willing to save. Batting scraps is a hard one for many of us. I don't do small projects or frankenbatts any more, I try to go ahead and throw away my excess batting that is smaller than a crib top.