The whole time you're masturbating, or just when you climax? Holding your breath while masturbating isn't any worse than holding your breath at any other moment. Even if you pass out, your body will just start breathing again, so it's not a big deal.
well im just worried about like... what if when im having sex, i can't climax unless i hold my breath?? that would suck.. it just seems like i cant orgasm if i dont hold my breath!
http://hipforums.com/forums/forumdisplay.php?f=13 might have more sources on this... it almost sounds (to me) like you may have a fetish (not a bad word! Just a thing that turns you on) for, well, not strangulation or suffocation but for being breathless, if that makes sense. But, I could easily be wrong. Personally, I usually take a deep breath of air before I cum, hold it for a few seconds, and then let it out in moans.
I much prefer lack of oxygen when I orgasm. I hold my breath when I masturbate and when I have sex... They don't notice.
I hold my breath too... no one has ever really noticed before (to my knowledge). I say if it feels good, do it.