Holding your breath completely is probably not the best idea, but if you exhale all your air on a heavy squat or deadlift, you're going to be losing intra-abdominal pressure and set yourself up for a back injury or worse. Like Eric said, controlled exhalation, like a karate "kiai", or a grunt, is the way to go.

Squat Rx #10: Set-Up & Breathing
http://youtube.com/watch?v=CXgEXiTCjqo