When playing Duration cards, I always place them over to the left, next to the deck. At the beginning of the next turn, I slide them over to the right, next to the discard pile, making sure to point out to newbies for the hundredth time that they are NOT in the discard pile yet. Yes it matters. And keep your damn green cards in your hand until clean up! Yes, that also matters!! Don't get me started. Anyway, all other actions/treasures get played in the middle.
This makes sense to me because so many durations do something at the beginning of the turn, including many that draw cards, so putting them by the draw pile seems logical. Likewise, the ones near the discard pile are "on their way" to being discarded and will get cleaned up with everything else.
Of course, Hireling and Champion hang out ABOVE the draw pile.
It's interesting what players see as logical ways to play out cards. I always play things left-to-right, but I've known players that, when they play +action cards, play their other actions below them like a binary tree so that they can visually see which actions have been used up, rather than counting. Fascinating.