sealing posts with TWP 100

My deck is elevated 5 feet above the ground.  Do I seal/stain them with TWP 100?  Down to the ground?  These posts are 40 years old but are in good condition.   The bad news is that they were set before sleeves were readily available.  The good news is 1) the substate is primarily sand so it is very well drained and 2) the prevailing breezes from the lake blow under and through the deck platform.  Additionally this space has had an open lattice screen around it.  This has kept leaves and snow (this is in Minnesota) from accumulating under the raised platform and the deck supports