I have been doing granny squares for over 30 years. I leave a long tail and when I crochet the squares together, I make sure to align the tails together and tie them into a small tight knot. I then weave the tails in. This might not be the official way, but I have never had a blanket unravel. If there is a tail by itself, I will knot it in place with a tight knot and weave it in. I think if you don't knot it, even a tail can can loosen out and unravel with washing and wear and tear. When the blanket gets older with wear and tear, the yarn will thin and loosen anyway, lending itself to more unraveling. I have granny squares that still good 30 years later because I used a smaller hook, crocheted the squares tightly and knotted them tightly.