I'd strongly suggest 100% cotton ONLY...I have been there. I bought a bunch of 50/50 poly/cotton. On sale..great deal etc. However, in quilting..that little bit of poly stretches.And it stretches just enough to make the square off measurement. It is also thinner than 100% cotton so you don't get the sturdiness you need for construction. I even tried spray startching it before cutting..helps for a little bit, but then..back to flimsey. I couldn't figure my problem on a quilt...all measurements were PERFECT...but that little bit of stretch of the poly was the problem.
However...having so much of hte blend...iti s great for making dresses for my little GD's.but..I use it often as I can make a match...I use it for the binding on quilts. The tiny bit of stretch that the poly gives is enough to accomodate the binding as it goes around the quilt. It is thinner so you don't get a thick binding as you might with 100% cotton. I do the way of cutting the binding twice a thick as you want it plus 1/2 inch and the fold in half, iron and then sew the raw edge side to the back of the quilt side and then fold the folded edge over to the front. Works great.