Definitely remember to breathe. A while back during multiple person Jiyuwaza (sp?) I used to get to about the fifth or sixth attack and run out of energy and become really dizzy, finally remembering to breathe. It's really silly as breathing isn't normally something you need to think about. It was the same after two hours of class when we do 20 break falls to finish off.
Your muscles need blood, your blood needs oxygen. Makes sense and prevents the dizzyness.