The enitre floor of the rear isn't really necessary. If you tap around with a mallet or something that can generate a rattle, you'll be able to tell where you need support, and where the metal is really thick. If you want to just do it all, no big deal...

The best method for the doors (IMO) is to cover the inner section of the outer door skin (reaching through the speaker opening when the speaker is removed) all around the speaker, then do the inner door skin completely, and cover the "mat" with a closed cell foam (like ensolite on Raamatt's page). You can also add some egg shell material behind hte speaker in the door to try and kill waves behind it, I've heard people have had good results with that. I myself didn't do it.

I have 2 12's in the hatch and did have rattleing issues with the strut tower bar (mostly the plastic trim itself), but without major bass you probably don't need to put anything on it.

There are 2 places in the rear that you really should hit. The back of hte hatch, that area rattles no-stop, and the areas over the wheel well. If you really wanna cut down on noises from rocks flying in the wheel well, they sell underbody stuff at the auto stores that cuts noise and protects...

If you just starting out, try doing one door and seeing if you can tell. My guess is that you'll notice a significant difference in the sound floor, and the response of the mid-bass driver.