I have 2 answers as to what I do with pieces of batting. They are great for quilt-as-you-go projects. I just made a dresden plate quilt by making the blocks, making the quilt sandwich, quilting them and then stitched them together. There are some good links on you tube for stitching them together and then...ready for binding. No one can tell I used scraps.
The other is fusible web. Rotary cut maybe 2 1/2 or 3 inches wide and lay batting edge to edge. Lay on the strips and , very important, cover with cloth and gently press...and you are done. If you don't cover the batting with a cloth, it melts on your iron...learned from experience.