Don't spam it. When I fight other Zeldas, it's my favorite bad habit to punish out of shield. Even if your shield isn't completely fresh, all you have to do is tilt it upward to avoid a shield stab and then enemy Zelda eats a bair/dsmash/dash attack out of shield. Pfft, ironically, I could even punish it with up smash if I wanted lol
Don't get too excited about up smashes when someone is above you and you know they still have their double jump -- all they have to do is double jump at the right moment and punish your lag with a dair. You might be able to out-prioritize them, but if you get baited, you get punished.
For dairs you know better than to try to beat like Link, TL, and G&W why bother risking it. Just bait it by standing where you know they want to hit you, then fox trot away and true pivot an f-smash where they'll land. Much safer.
If you're going to use it for shield pressure, you have to make sure it will stab their shield before you use it. Use Din's to weaken their shield if they shield that. Preceed it with a jab at shield safe distance. Better yet, if you can d-tilt and jab first and they still can't figure out a good out of shield option, you've got a good chance of either shield stabbing them or breaking their shield. If they do reveal their out of shield tactic to you, at least you didn't have to commit to a laggy attack.
Jump canceled up smash out of shield is your fastest out of shield option. If someone is pressuring your shield at close distance like a Peach (floated dair to nair) or a Fox (dair to grab) might, this will take full advantage of your frame advantage. Also punishes a Marth who very poorly spaces his fair -- nair oos is better for less poorly spaced fairs, though.
Up smash is great for punishing spot dodgers. If someone is spot dodging twice in a row a lot or just spot dodging predictably, go to town with your up smash. Works on air dodgers most of the time, too.
It really shouldn't be used as an approach in most situations. A hyphen smash from one point to the other has about as much range as your fsmash. Use it with set-ups like jabs or d-tilt, to juggle at 0%, but the majority of the time it's best used defensively.