I've made 4 flannel quilts and I used flannel for the batting - the flannel batting makes the whole quilt "supple" - no stiffness to it even after heavy/dense quilting.
I've made 1 with just squares (placed diagonally in color match), 2 with squares and rectangles, and one with squares and rectangles on point so there were setting triangles involved and I added a diamond border. Pic of the last is below.
Did not have undue problems with stretching, and it was a bit of a bonus when I had to do the border to get it to meet right at the corners :)
I pre-washed and dried a few times before I cut.