Have a look at this site:

http://www.quarter-wave.com/index.html

Well I would say as far as placement within the mounting surface goes, centered would be the way to go. My reasoning for "centered" is that if not mounted that way you would probably end up with some whacky, uneven resonance on one side of the box more than the other side. Who knows what that will do but I can't say I have ever seen a woofer mounted off center. In terms of where to "fire" the woofer itself, you are in a pretty good position to experiment since you are building the box yourself. I would wait until after the main box was built before deciding on how to fire it. You could use some cheap isolation blocks to rest the box on and see which way offers the best response in the room you plan to use it In. As far as my understanding goes downward firing the woofer would boost bass response in the room if you don't have any nearby walls to leverage for this purpose. If bass is too bloated in this configuration then you can try forward firing the woofer and see if that evens things out. Once the best way is determined, use the isolation blocks between the floor and the box itself just for better isolation from rattles.