It depends on the reason you are wearing a necker.
For a user, handle down in a secure sheath is the best choice. If the knife will not work well in a very snug sheath, then handle up works. You grab the handle from the chest side and rotate it downward as you pull it out,
For minis and display purposes, as is often seen in a knife show, the neckers are worn like a bolo tie, and are almost always handle up. These are rarely used to cut anything and are generally knife accessory wear.