Depends on who it is and if the fight is even worth it. Normally I'd fight anyone who throws a first punch, but if they're just pathetic and not worth my time, I'd prefer to walk away and leave them screaming in frustration.
It depends entirely on the circumstances. I have never fought with anyone as an adult except my ex. As a child, I fought with my much bigger sister and one boy that I almost drowned in a ditch. I think fighting is uncivilized. It makes the participates look like animals!
If it's someone trying to start something with me I refuse.