I haven't sewn with denim, but I did use some thick decorator fabric in two quilts. I just used bleached muslin without any backing as sashing, and had no problems with the difference in weight. In fact I think that not having seams where two heavy fabrics were joined made the quilt go together much more smoothly, and the seams lay flatter than they would have with two heavy fabrics. The differences in fabric weight weren't visible once the quilt was quilted.