While concrete does help in stabilizing a fence line it is not 100% necessary. If you live out at the beach, near the inter-coastal waterways, or anywhere there is sugar sand or high wind volumes, then yes you may consider concrete. If you live closer to downtown, Julington Creek, Northside or the Westside of town, you are more than likely safe to install directly into the dirt. After the concrete has set it forms a "jug" around the post, the jug allows for water to collect on the "top shelf". The water sitting on the shelf will cause the post to rot quicker than if the post was directly installed into the dirt. When the post is installed directly into the dirt, the water can flush back into the ground without incident to the post. Keep in mind if you are choosing to do Aluminum or PVC vinyl fence those styles are always concreted without an option.