Best way for boxer to breathe?

[cocobeware]

I have been told before that it is best for boxers to breathe in and out through the nose keeping the mouth shut and biting down on the mouthpiece and also to practice this when doing conditioning work like road work and sprints ect. However i have always found this difficult.
Should i try and force myself to breath using only my nose? I know my running times and boxing performance would suffer greatly in the begining but would i benefit in the long run? I know that a boxer getting caught with his jaw hanging open could be disasterous.