Well...in martial arts we breathe out at the point of maximum physical exertion.
Amongst other things, this helps tighten the core and helps prevent fatigue.
So when PowerBocking, if you followed this rule, you would breathe out as you
compressed your springs.
...But like the guys above, I've never actually thought about it while bocking.
Breathing is a natural process dude. Holding your breath is what requires thought.
I hope this advice helps though. =) ...The short answer would be:
If you are jumping for much more than a minute, breathing is very important! ^_^