Nancy I haven't used fleece for a rag quilt before but I do use flannel which works very well. I use a square of flannel as batting between a top and bottom flannel square. If you cut the batting square slightly smaller it will make clipping your seam easier but I actually prefer the finished look of a thicker ragged effect so I make my batting the same size as the other squares. You can also make the batting square out of a contrasting color which will show in the ragged edge.

Are you planning for your quilt top and back to both be fleece or one side fabric and one side fleece? Fleece is quite a bit thicker than flannel so making a 3 layer sandwich might be harder to work with in fleece. I think Sandy is right about not needing a batting in that case.