one of the reasons (traditionalists) say to not use sheets is because of the thread count- sheets tend to have a higher thread count than the fabrics we (normally) use for quilting- and can be a bit more difficult to stitch-
especially if hand piecing or quilting- but more and more people are using them- it is your project- you can use what ever you want- no one even needs to know it was a sheet-
but if you are going to do hand work- be prepared for it to be a little more difficult