I've become quite passionate about building fences. I live in northern alberta where the frost goes down deep. I always use 6x6 posts unless the HO's budget is a bit wimpy, otherwise 4x6 minimum or I turn the job down. When I'm setting posts, I drill 54" to allow for a 6" base of washed rock or gravel. Before pouring concrete, I will install a small piece of 12" sonotube at the top of the hole if it is augered out really wide due to roots or loose soil. If you keep your "finished pyramid" within the diameter of the post hole, there is no chance of the frost heaving the post. It's when you fill that top augered out part with concrete, is when you will get frost heaving. If you create a concrete mushroom, the frost will have something to push against. As for the rot issue, I always stain the framing right down to the top of the concrete before backfilling. Another layer of protection above the treatment never hurts and I've never had any problems.If I did that in the area I am in, the posts would all be heaved from frost in less than 2 years.
I've removed many, many crooked fence posts/fences because the previous installer had the concrete up to ground level.