get your system in, play it full blast, narrow down the problem areas and buy the amount needed. You mainly only want to use it on big flat areas that can vibrate/flex to the point where it makes noise. You put CLD/dynamat/deadener matts on to reduce vibration and lower resonant frequencies. Less flex = less energy loss = louder. Not by a whole lot but its noticeable. You dont want to just cover up every nook and cranny, thats a waste of money. However if you plan on doing the whole car, look up stinger expert roadkill 36 square feet pack under 100 bucks and its thick and sticks pretty well.
Most other rattle problems, you should use weather strip foam, expanding foam aka great stuff(red can) and closed cell foam. Its to decouple(pad the surfaces, keep em from vibrating against eachother).