We like to use screws to fasten our 2×4’s to the posts instead of nails because it makes the fence stronger. Where to post and 2×4 meet is where the brunt of the wind is felt. Also, over time wood will dry out. As it does, nails offer no strength compared to a screw thats threaded onto the wood. A nail is only wedged in. Other companies do not use screws because it takes more time, screws are more expensive than nails and it requires another set of tools.