As with anything in life, using the correct tool for the job is important.
There seems to still be a lot of confusion over the purpose of sound deadener, and what it should be used for. That's ok though, it does have its uses but it shouldn't be the only tool used to help insulate the cabin.
There are typically three types of acoustical noises that would happen within a car's cabin:
1) Resonance
2) Rattles
3) I don't have a good word for this one, so we'll call it Environmental noises (think road/tire/wind noise).
Resonance is the energy from some source being transferred to another object, causing it to vibrate. Think of this as the skin on a drum, if you hit it with a drum stick the whole skin will resonate. Now of course with that being the intent of a drum, it isn't an issue. But think if you were to hit one drum, and then the one right next to it also produced a sound. It wouldn't sound proper, right?
This is where sound deadener comes into play. In a car, replace the first drum with your speaker, and the second drum with the panel it is attached to. The speaker causing resonance (which we interpret as sound) is perfectly normal. The vibrations from the speaker causing the panel to also vibrate is bad, it introduces unwanted sound. There used to be a great resource (RIP sounddeadenershowdown.com, actually it looks like it's back up and running so that's awesome) that covered the efficiency of sound deadener, but in short 25% coverage of any panel is sufficient to reduce resonance to an inaudible level. You don't need to go crazy, it works I promise. Plus it saves weight.
Rattles can sometimes be caused by panel resonance, but not always though they have similar causes. Rattles are when two separate panels are touching each other without an insulation barrier between them. Pretty simple fix, find the rattle, and decouple the panels using some sort of Closed Cell Foam around 1/8" thickness is usually enough. CCF is important because it wont absorb moisture.
Environmental noises actually require something to block the sound and insulate you from the world. You should do the first two before you jump this far ahead. You need Mass Loaded Vinyl for this one. MLV is generally used as a commercial noise insulator, but it works in cars, too. You need about 1/lb per sq/ft of weight to be the best balance of effectiveness and weight and it needs to be flexible enough to contour to the area you're installing it in. You'll want to lay CCF down first and then adhere the MLV to the foam to prevent the MLV from rattling against anything. While it definitely works, the downside is you really need to do the whole floorboard to get the best results. If you only do the doors or hatch area, the environmental noises will just come in from somewhere else. Of course, how crazy you want to go is up to you, and you definitely can get results with targeted applications.
Hope this helps!