I use flannel sheets all the time. There is no reason not to use them and they are warm, cozy and tend not to slide off the bed as easily. They will shrink, so wash and dry before using unless you like the slightly antique puckered look. Some flannels are not as tight a weave and may stretch slightly so watch out for puckers on the back if machine quilting. Some flannels may also fuzz or pill a little after washing. If using flannel with a Warm and Natural Batting, you will have a heavier warmer quilt so consider what is your end use. Flannel backed quilts are great for Canadian winters but may not be appreciated in Florida.