I made some denim/flannel rag quilts - but I used the heavy denim and chamois type flannel for them - I cut 5-inch squares - I thought that was dense enough.
Denim comes in a lot of weights - I did start to baste all the squares cut from one pair of jeans together ("bundled" them) so that when I laid the squares out I could find "matches" easily. (I know - just a touch of obsessive-compulsive showing there)