I have tried making string blocks many different ways over the years. When I don't use a foundation, they wiggle out of square for me, even when I make them oversized and then cut them with a square. A piece of thin muslin works although the result is a somewhat heavier quilt. I now usually use foundation paper (newsprint, old telephone book pages, etc.) even though I hate removing the paper. The unavoidable bias edges just seem to need a bit of shoring up.