A semi-crouch is okay, but one thing that you want to avoid is putting your weight on your front leg when you're not punching. This will make you end up taking the full force of your opponent's punches and take the power out of your own shots. Have you seen Juan Diaz? He can throw punches like a hurricane but he couldn't crack an egg with his shots. This is due to the most part because a lot of his weight was usually kept on his front leg, everything in his posture was working against him.

If you understand a little something about power you'll know that it's about shifting your weight from one leg to another, but most the time you should be pretty balanced.