I also use whatever I find in the scrap bin, but sometimes I have to consider the quilt design. If the intent is to hang on the wall, then I try to use the cotton batting to keep it a bit stiffer. If it has a lot of pieces and I plan to do a lot of stitching on it, then maybe the thinner bat will do. If it has a lot of applique, a puffy bat makes it look awkward and stiff in places. So I guess it's just personal preference or whatever's on hand.