Never put a post for a deck or porch into a concrete setting. Wood is constantly receiving and expelling moisture causing it to expand and contract. This results in a crack or space between the wood and concrete. This allows moisture to get down to the bottom of your footing. moisture can get in quickly but takes a long time to dry out, if ever.
This will cause your post to prematurely rot from the bottom. This is your main support for your project!
Instead, pour your concrete footing and use a "post anchor" to hold the bottom of your post up off the ground. This way, if it gets wet, it will be able to dry out in quick order.
This tip will ensure you get the maximum life out of your posts!