I've been working with some donated poly/cotton lately and did not encounter any of the problems that have been posted. Kept my iron on a low enough setting.... pinned my sandwiches about every 4" ... the tops were poly cotton scrap pieces that had been donated.... then I pieced my batting with warm and natural cotton scraps.... then I used polyester fleece chunks that I had salvaged from previous backings...pieced them together....
http://www.quiltingboard.com/picture...t-t264309.html
PS... usually I use 100% quilt shop quality fabrics in my quilts but that doesn't mean that you can't use whats available or affordable if you don't have those