I do both depending on the distance. A hook from a bit further away for me feels better with the palm facing down, a hook from inside feels more natural with the palm facing me. I had a broken wrist when I was young on the right hand so I can't get it facing myself properly, I throw that one at a 45 degree angle at best. But whatever connects with his face counts, it doesn't matter much.
there is heaps of ways you can throw the left hook but palm down is better for sinking the knuckle in.