I tried Cabusi's described method of pressing the camera trigger/shutter while releasing my breath. I found that my chest was contracting, making my camera move slowly downward, causing my camera to be a bit less stable. That difference could easily be enough to blur my image when using slower shutter setttings.
Hearts will always beat, so that is a constant that is not changable for most living people, so I don't see how holding one's breath while pressing the shutter would hinder the quality in any way. By holding the breath, I have basically eliminated one element that tends to make my camera move.
I did not hear a reason for why it could be a bad idea to hold one's breath while shooting an M-16 Rifle. Maybe doing so while feeling the kick of such a rifle could be a health risk of some kind.