it's not NECESSARY, but it's sooo soo nice. deadening your doors especially. you get double benefits. cutting roadnoise, AND enhancing your midbass responce of your door speakers. Overally I couldn't really say if it's worth it or not, sorta all a personal prefrence thing. I for one think it is. BUt there's others who strip every etra bit of anyhting whod find adding mass to your ca rintentionally to be silly.
Doors are a good place to start, if you like the effect, try the carpet padding thing, ESPECIALLY over your wheelwells. makes a HUGE difference. other ppl's 5's are annyoing for me to ride in now that my chick and I have both done it.