If you decide to fill them only fill the ones that are facing up. Put a backer foam in the crack and then fill it with Energy Seal. You will need to inspect these twice every year for separation from the wood. If a hair line crack appears repair it immediately. Water will seep down in behind the caulk and not be able to dry out fast enough and cause log rot. Typically I would not recommend filling checks, but it sounds like you have some pretty large ones.You can get the materials that you need from Timeless Wood Care Products at 800-564-2987.