Preffered firing direction of sub box

I was wondering what is the preffered direction of the members of this forum for the sub to fire. I'm sure it depends on the install, but what I am wondering does the traditional rear firing box the most common, do some prefer the speaker firing up towards the roof of the trunk / car, or do you prefer a box built in a side compartment (like the area between the top of the wheel well and the rear of the trunk). I am wondering what has given you the best performance, as i'm sure it is also dependant on what type of vehicle its in.

Thanks

This is the article I've seen referenced in a few different places regarding this topic:  http://www.inhttp://www.installer.com/tech/aiming.html.  This is for trunked cars and one suv, and did a forward and backward-facing but I wish he would have done up-facing as well as that is what I'm most curious about.

Not to hijack the thread but for you of those out there with experience what do you prefer in SUV's because I'm about to mount my subs and port facing upwards in my cherokee (due to aesthetics and size constraints) unless people have had significantly better results with rear facing. Any opinions?  Oh yeah, can't try both because the box I build is going to be a tight fit.

stevdart, could you explain this a little more for me?  How does the gravity make more wear and tear on the subwoofer?  Would it be that, say if it's upfiring, you would have the down force of the motor plus that of gravity taxing the suspension more?  What kind of decrease in lifespan is normal?  If this is a big deal I'm going to have to redesign my box, DANG!