I used a sheet from Target for backing on my daughters wedding quilt in Oct. '07.....it worked out very well.
The LAQ told me to open all the seams and the top hem to give her more length ......I've been using sheets off and on for many years for backing and guess they work out ok.
I've only had the one quilt done by the LAQ.....I do my own on my home machine. Nothing fancy but it holds the quilts together
and I make lots of quilts for charity and gifts and orders from friends ......
If the color of the sheet is right I sometimes use them for quilt squares for a simple basic quilt.