Concrete wood posts or not

The fence companies that install a quality product do poured concrete footings on each post. Leaving the lower 10" to 12" bottom of the post tamped in with dirt or gravel and pouring a concrete footing for the balance that will give you the best result.  Sterling Fence Inc. uses a 9' long post on a 6' high wood fence to leave 30" to 36" of post in the ground.