It totally depends on both the climate and the type of wood. There's no way to say what works best in a general way. Whatever it takes to get the wood to about 20% moisture content is what's best. White Ash stands green at about that figure so it'll burn the same day it's cut. Take something like Apple or just about any Hickory, and you're going to need a full year to get the most out of it here in the middle of Indiana. Sure, it'll burn sooner, but it's wasteful to do it that way.
I've only been burning wood for heat for a couple of decades so I leave room to learn more about it, but in my experience, wood does not even <i>start</i> to season until it's been split and stacked. Covered is okay so long as full air circulation is available; uncovered is fine too.
Don't forget to fetch <a href="http://www.arboristsite.com/attachment.php?postid=165043">this PDF document</a> attached to a post of mine in the "Cord of wood" thread here. It has data about moisture content and weight of a bunch of different midwest fuelwoods.
Glen