I aint trying to descredit anyone here, but eaven if you put dry concret in a post hole it will get hard, and even with treated wood, wood will rot if water has no way to get out! and with expansion and contraction wich all wood will do it make's a space between the wood and the cement at one time or the other, heck if its dry it may even split your footing if gets wet.
There aint no dout, water will hole up next to the wood when it rains. and pea gravel is a good footing for a post for the water to escape. over here we go down at least 42" and pack the footing and we warenty the work with out much worry.