I have done both, it just depends upon the quilt, it's purpose, where it is going and my budget. If I have the coordinating fabric, I love to do a little piecing in the back to add interest and 'tie it together'. If I am pressed for time or budget constrained, I like to buy used cotton sheets from the thrift store and use them for a back--can't beat $3 for a back. Other times if I don't have any inspiration, I use a wide muslin backing with no seams.