In an enclosed vehicle, speaker placement can very much affect which method would be best for the situation. The problem is like this.
If you have a box in the trunk of a car, pointed up to the rear deck into the cabin. You're bass responce is going to be horrible. Why? Because you get a reflective bounce off the back wall of the trunk which comes out the rear deck off-phase of the main wave and cancels the sound. Pointing the speakers to the back in the trunk results in only the bounce wave going through the deck, hence, more and better sound. Now the front-firing would work if you sealed off the face of the box from the rear of the trunk in some manner so the wave could not travle there... (making sence?)
Now in the open cabin of an SUV, it's a little different. If you want the speakers right behind the rear seat, then rear-facing or down-firing are usually the best options. If you want them by the back wall, then forward, up or down-firing again work best. There's a second catch to this part though that the bounce wave can more fully develope, effectively resulting in more bass output with rear-firing, mid-sub setups.
Here's a good article to help explain some of this...
http://www.installer.com/tech/aiming.html
Hope that helps make some sence.
