lots of poly threads are just fine to use...i would only worry about content if i were making some big extreme heirloom that i felt i had to boast ... 100% cotton, even the thread.
i generally use cotton/poly blends they are stronger, i get frustrated with cotton thread breaking all the time. i use cotton thread if i'm hand stitching but usually i don't even pay any attention. when i choose thread i choose by color and weight. and personally love using silk thread, but it is expensive and some people don't think you should use it for cotton fabrics but i've not had any problems and it is a dream to sew with.