When taking a breath, it is easier to breathe through the mouth, it is quicker and more efficient. There are merits for breathing through the nose, but it is usually a more advanced way of breathing. It is not a way most orchestral players breathe. Breathing in though the corners the of the mouth is the correct way. Make sure that you play 'at the top of your breath' i.e. take the biggest breath and don't hold your air before you play. Breathe in, blow straight out. You will find this minimises tension.