My mom taught me to knit, and she told me how to wind yarn into balls without causing it to stretch from the tension. It’s simple, really. Just wind the yarn over the fingers holding the ball. Slip your fingers out from under when you switch direction, then resume winding over the fingers again, till the ball is finished. That’s all there is to it. I’ve always done it this way, and find that it really does prevent yarn stretching, even when stored for a long time.