**No! It is utter rubbish.**

It is one of the common tropes that gets totted by deluded people who claim to know anything about self defence. It reads sensibly in the face of it but as soon as you think about it in depth, its sheer stupidity comes to the front.

*I am not lawyer, nor do I play one on TV*.

Acting first makes one the attacker. The situation was escalated form an argument (possibly loud and obnoxious) into a physical confrontation. Any LEO will arrest whoever started threw the first punch. Prosecutors will argue that the attacker is a violent individual and Judges will listen to them. After all, the attacker does a violent sport those aim is hurting people -- a skewed definition of martial arts.

Self defence does not give one unbridled rights to hurt, main, cripple, or kill one's attacker. What the legal definition of self defence is varies from country to country. If you want to know more, talk to your local lawyer/solicitor.

In addition, people who rely on either violence or the threat of violence to resolve any confrontation are called bullies. It makes one a despicable person most people do not want to associate with.

However, even if it were a good idea legally, the statement would still be utter rubbish form a training point of view: no one walks around expecting hidden and invisible ninja attacking them at any moment. It is impossible. Even soldiers in the middle of battle have trouble detecting attacks. And they are trained for combat. After some time, PTSD starts affecting them -- or combat fatigue, whatever you want to call it. 

Let's assume you want to go down that path.  You have to watch for ambushes, expect the worst of all strangers, and watch out for hidden dangers all around you. This is what the medical profession called [paranoia][1].

Now, in your paranoid state you thought someone is coming close to you, hands in pockets. Is he going to stab you? Yes, quiet possibly. So, react first: pick you gun and preventively shot him in the face! What if you get it wrong? What if that guy just did not see you and bummed into you. He was thinking about his wife and what she wanted for their anniversary, just not paying attention to his surroundings...

Is that how you want to live? Always afraid for your life, never being able to stop and rest. What kind of life is that?...

Of course, you can come up with lots and lots of imaginary situations where this advice saves your live. But they remain *imaginary*. In real life, this advice is utterly useless.

  [1]: http://www.oxforddictionaries.com/definition/english/paranoia