I too use sheets all the time for quilt backs. They are absolutely not problem for machine quilting and wash and wear very well. I especially like them on quilts that will see a lot of hard use (think Youth Shelter and industrial washing machines) or quilts for kids that will be really used and dragged around. I frequently buy 200 threat count sheets ($5.00 on special in Canada) for Community Quilts and even higher thread count sheets on family quilts. I try to buy the darker colours or patterned ones in King Size and use the leftovers to make a matching pillow case to store the quilt in or as the gift bag to present the quilt. Using sheets really cuts down on the overall cost of a quilt and I can't think of any reason not to.