All depends on what it's made of and what you are hanging it from.

The looser the knit, the more damage hanging will cause. A hook might cause more damage than a hanger.

If we're talking about the typical hoodie made of sweatshirt-type fabric, with cotton and maybe a little spandex, hanging most likely won't hurt it. There might be a "bump" in the fabric where the hook or the end of the hanger was, but washing and drying the hoodie will make that disappear.

But if you're thinking of hanging a good wool sweater--well, those can stretch out if hung too long and they might not go back to their original shape. So for a fine cotton or wool sweater, I'd fold it and store it in a drawer or on a shelf.

The more you can stretch a knit fabric, with or without spandex, the greater the risk of harming it by hanging.

I'm in total agreement with Camlan. The only thing I want to add is that a padded hanger can prevent "bumps" on some items. I hang my velour hoodies on the curved clear plastic hangers and have not had a problem. I hang some good sweaters and keep others in a drawer; it depends on the tightness of the knit.