I would use them (and I do at times in scrap quilts) with these precautions. I wash everything before I use it so there's not a problem with fabrics shrinking differently. I also test with an iron and if it shrinks up with a hot iron, I won't use it. And I also make sure it's the same weight, for instance, not a heavy poly or blend with lightweight cotton. Also, if its a more open weave, I will toss it. And be careful with swaps as most people want only 100% cotton for their swaps. My grandmother used anything and everything, including poly blends which have been around quite a while, and her quilts have all been used, loved, washed, etc. for years and had no problems with them. And she hand quilted almost all of her quilts. She even made quilts with double-knit polyester but those were just utility quilts and were tied and given to kids usually.