For some reason when lifting many people forget how to breath you see it on squats, deadlifts and bench press, purple faces and people hold their breath through the lift, This is not good for you at all not only does it increase blood pressure but it cuts of oxygen to the muscles essential you could be making yourself weaker
Start each set with a deep inhalation and exhale as you push through the most difficult part of the lift"
I think holding your breath through the rep and breathing at the finish of the rep is a personal choice. You should check out Rippetoes SS book as he discusses the benefits of the Valsalva maneuver. I use that technique and it has worked for me absolutely perfectly. If anything I would exhale AFTER the hardest part of the rep.