well im not too sure, but im quiet confident in saying that you should never hold your breath when working out, just breath what feels most comfortable to you.
when lifting weights a lot of people say breath in on the easy part of the lift and breath out on the hard part, for example when bench pressing, inhale when lowering the bar and exhale when pressing it up. i personaly never breath like this, i just make sure i keep my breathing constant and make sure i never hold my breath, as this feels most natural to me.
so breath the way that feels most natural to you, and never hold your breath.
well thats what i think any way, not sure if its right.