Waiting to exhale

I only have a minute, but I have a question that has been in my mind for a while. I've been taking exercise classes and the instructor, like all the other ones I've had, tells us to exhale on exertion and inhale on the negative. How important is this?

Sometimes I feel like I'm hyperventilating if I do this and I don't need to breathe that often. Other times, my respirations are off -rhythm, but it doesn't bother me. So should I breathe when it feels like I need to or do as the instructor says?

BTW, I've found that although I'm very weak, my cardio fitness isn't that bad and my balance is really good. I did great on the Bosu ball, but I had trouble climbing stairs for a few days after! I'm really enjoying this class. It changes each time. I'm looking forward to our lifting day. Today we did Boot Camp but it felt more like a football practice. I'm pretty sure I won't be able to get out of bed tomorrow.