If we were meant to relax before doing anything exciting we wouldn't be human. Its a built in response.
You see a woman/boy/husband/wife/girlfriend/boyfriend you want, heart pumps faster, you don't all the sudden attain the lotus pose and chant OM. I believe Bruce Lee in his writings called it our "killer instinct". Of course he also had you relaxing your punch until it was 3" into the target which unfortunately is metaphorically correct, but physically a bowl of jelly. He just wanted you to punch farther and used that to help people do it. Watch some video and you'll see he has max muscle tension before the fist/foot goes through anything. Fluidity is not relaxation, its fluidity.
Sure can you learn to relax? Absolutely. But why? Tony Blauer has an entire self defense program based on the flinch reflex, and we all know that isn't relaxing. Funny thing is as far as any program I've seen in some time, it actually has very definite real-world application, can be understood quickly and efficiently, and uses something we already know how to do.
Relaxing is great for mental clarity when the body isn't doing anything. When the body is doing something, I agree with Garrett get the blood and adrenaline pumping and kick some ass.