if a fight is going to happen (usually there are signs before it does such as verbal, posturing etc) then in all honesty you are better off getting the first one in.
I know all the legal buffs here are going to say "oh but you will get done for assualt etc" which may be the case depending on if there are any witnesses at the time etc but you greatly improve your chances of walking away in good condition.
Action is always faster than reaction, it is a fact that can't be argued. remember back to the school yard when you used to play games like 'slap hands etc'. It always sucked being the person getting slapped didn't it?
You can still act in self defense and throw the first punch.
Sorry my skin was getting all wrinkly in the shower....