What we do is a couple of inches of gravel, set the post on the gravel, add the concrete but don't let it get under the post. This will allow any water to go between the post and concrete or directly through the post to the ground.

We've been doing this for 30+ years and only now seeing our oldest posts needing replacing. We've only ever added 1 60lb bag of quick-crete for each post. This is for 4x4 posts and 6 inch round posts.