I have not been a fan of using sheets for backings.   Not that I have ever used a sheet as a backing, so no real experience , but only because I’ve always heard that its not a good idea.   Well I had a customer want to bring me two  t-shirt quilts and use sheets for backings.  She said that she had read my page and that I didn’t recommend using sheets, but ask if I would make an exception and use them for her.   She assured me that she had used them before and never had an issue with them.   So I told her that I would use them, but I had never actually used sheets – she trusted me and said she was ok if they didn’t come out perfect.  Well, I have since quilted about 6 quilts for her, using sheets for everyone of them.  I didn’t have any issues and they came out wonderful.   She was so excited how they turned out.  So not sure if I would recommend a sheet for everyone to use, or to use on every quilt – but I’m not opposed to them and will use again in the future.  But I would recommend getting a good quality high thread count sheet.   And of course make sure its 3-4 inches bigger than your quilt on all four sides.   Happy piecing!!!!!