Truer words have never been said. It does so many things, none of which are detrimental to your sound system and only serve to make it more efficient. Some cars are designed for a quiet interior, those are the best to work on. I don't need to deaden anything but my doors, trunk, and the parts where I ripped the interior out of for weight purposesProper sound deadening will really bring alive your set up !
Start with your doors and your trunk (or wherever your subs are mounted.better sound to start with