Here goes............
The reason ours don't warp or bow is because we are not painting them with latex or oil based paint. Over the years, here at s101, I've made special effort to emphasize 'solid color stain'. It's almost like a paint, but it's a stain and allows the wood to breath. Therefore, you're not trapping any moisture in the post/wood allowing it to continue to breathe, if you will. Checking, blistering and twisting occurs when you trap moisture in the wood and it moves horribly to get out, by going back to it's original shape.
Another thing is to look for double kiln dried posts. These will almost be guaranteed not to warp.
The rounded posts are generally 8'ers and are for landscaping, but if all you need is 8', by all means get them. They're usually cheaper.
One thing to add, almost all treated posts are going to get some bow or warp into them, eventually. These trees are scientifically cut down so that even if a gnarled sapling 40' tall is 4" thick most of the way up, they'll still cut it down. In the saw mill, they have saws hooked up to a computer which will cut a perfect 3-1/2" x 3-1/2" all in one shot straight up the tree. The trees are soaked down so much they almost flatten out, but the specialty saws can account for the bends and twists and after being cut, pressure treated and stacked, they will remain rather straight. As they dry they might go back to some warping a little bit, but once you add your paint... the process is sped up beyond belief and they'll warp within weeks and try to go back to their original shape while they were still alive and growing. Many years ago, a good friend of mine went on a tour of one of these companies and he told me how amazing this process is/was.
