We may not be Jones Jr. or Mayweather, but that's ok we're not fighting their competition either.
I'm convinced after thinking about it. My right hook is more powerful when thrown with the thumb up against a bag, but I can't land it so it just stinks in sparring. With the thumb towards me I can get it off faster and hide the punch better, Hopkins style. Landed a few of them today actually, which surprised me.
Same for the jab, I sometimes throw it with the thumb up. Salvador Sanchez does that a lot, and so does McCallum. I'm being told not to do it, but biomechanically it just feels so nice in certain circumstances.
I'll be doing both from now on, and no longer bother myself with wondering what the "correct" way is. The correct way is when it lands on your opponents chin and bothers him.