There are so many different types of subwoofer on the market, that the only place for it is where it sounds best....on the floor is a good starting point though lol
depending on if the ports are rear/forward/side facing will decide weather it goes close to a wall, for example, a rear ported sub can sound muffled and boomy if its butted right up against a wall, some subs (and speakers for that matter) sound better with a bit of space.
Just experiment, youl be suprised at how you can influence the sound by moving the speakers a bit.
after all, what sounds best to you is always the best solution.