I've made a few of these and the seams, hold up great! I've made one for our bed (queen size), plus a denim and one for my Halo finatic son...they've all held up wonderfully!
I used homespun fabric and did prewash all that fabric, but the flannel and cotton I didn't.
I sandwiched 3 extra pieces of flannel between my denim quilt for extra ragginess!. *L*