It will depend on what you're feeding them. Chick starter doesn't have enough niacin for ducklings, since it's made for chicks, so if that's the only food you can find for them, you will need to add niacin. A flock raiser feed, or a duckling specific feed (which is hard to find depending on your area), should have enough niacin already it in, meaning you likely won't have to add more.
However, the needs of individual ducklings varies, so while some may be fine eating a flock raiser feed and not need more niacin, some may require more and may still need it added to their feed, so it's not a bad thing to have on hand just in case. Brewer's yeast is a good source of niacin that you could pick up and have if you're going to be feeding chick starter or just to have in case a duckling does need more niacin.