This is one skill which is very critical prior to an interview.
Yes try to relax before an interview, this is very critical specially when you are waiting for your turn to come up. Specially when getting that job is very critical. This will help you a lot, the reason plain and simple. How many times have you made a mistake in life when you were stressed, in a hurry, nervous, etc. Well personally I would have made more mistake then when I was relaxed.