If it were me, I'd probably keep only the 100% cotton and give away all the rest. I could be talked into using a big piece of poly cotton blend for a quilt back, but not for piecing, and never for applique. I don't even buy clothes which need dry cleaned, so I wouldn't be interested in pillow covers which needed such treatment. But, as already pointed out, you have nothing to lose by washing some and seeing what happens. I would certainly recommend washing all pieces before putting any time and effort into piecing with them, only to see them ruined later.