Whenever you think "I should use down-b": Don't. Use neutral B instead. Down-B is a very situational move. IMO, the only times you should use it are:
-Edgehogging
-Feint a dash approach, when the other player anticipates and reacts with a shield, grab or dodge, down-b
-When the opponent approaches you from the air, you have not used it in a while AND has high percent. Down-b is a finisher.
-When an opponent is trying to dash at you with a dash attack or up-smash out of a dash. Just hit forward then back and A, and you should move slightly backwards and attack in the same direction you originally faced. I've gotten my friends a few times with that for a kill.
Down-b should be used about once or twice a match if at all. I would say use it more of you were invincible while invisible, but you're not. So it mostly sucks. Neutral B is much better.