In Thomas Mark's book, he addresses the subject of breath by saying that you can either phrase your breath, or you can breathe naturally. It is whatever feels the best for you.
I have taken it to the next level and now practice a breath relaxation exercise called Heartmath. It hooks up to your computer, and you can do it for five minutes a day or twenty minutes, whatever you like.
What it does is to train you to gently focus on your breath for a specific period of time. After a while, you can "zone in" on it while you are at the piano, and essentially turn it on or off. This also comes in quite handy when you are in other possible stressful situations.
Further, if you are holding your breath, and brother I have been there, you are probably tightening your stomach and also clenching your teeth. That is why I recommend an overall exercise like the one above. You cant make it work without your whole body being relaxed.