i often recycle and re-use fabrics...but if they are thin/fine they really are not suited for quilting- unless you add an interfacing to them to give them some body/substance- they will be too fragile & chances are simply fall apart with the first laundering. if the fabrics are good quality, not too thin, fine they will be perfectly ok to use. sheets are generally not recommended for a number of reasons, the thread counts are much more than the quilting fabrics we use-so they are not as easy to work with. you can use spray sizing or starch to make your recycled fabrics nicer to work with= but if they are thin/worn to the point that a spritz of starch does not give them body- they should be set aside for some other use (like to use for practicing/checking tensions/trying new stitches, a press cloth, or even foundations for 'paper *foundation* piecing...or if you love them & really really want to use them purchase some lightweight fusable interfacing *woven or non-woven* (not paper backed fusable for applique)and fuse it to the back of the fabrics...then use as desired.