Fudging your rolls, or essentially lying about what you have, be it the dice rolls or a character’s stats. While it is universally agreed upon that players are not allowed to lie about their rolls or stats the narrator has always existed in a sort of grey zone. The narrator can often roll behind a screen or similar object to hide their rolls. Because of this it is easy to lie about what you rolled. Some narrators believe in being completely open and will roll in the open so everyone knows what’s being rolled. This establishes additional trust and makes it much harder for players complain about rolls they think are unfair. Along these lines the narrator should also decide if they make the npc stats known to the players. If they do then players know how much they need to do to bring down their enemies. This also means that if the enemy is killing the party they know that you’re not lying about the rolls or the stats of what they’re fighting.
On the other hand, there is always something to be said for the narrator operating in the shadows. When the narrator keeps their rolls and stats hidden then it allows them more fluid control of the story. There are a lot of reasons to do this such as keeping the party alive. If the party is fighting a monster that you made and are getting destroyed by it then as the narrator you could make some on-the-fly adjustments to the creatures stats so that the party is not killed by it. Or if it hits one of them in a way that would kill them you could fudge the roll so that it leaves them barely alive. This is one of the powers that can be abused the easiest as a bad narrator will completely forgo stats and decide when the enemy hits and when it misses as well as when the party hits and when they miss making the battle pointless. But a good narrator can use it to craft a more riveting story. Instead of killing a character perhaps something important to them is destroyed or a limb is lost. If the enemy is too strong subtly lower their stats or say an attack from the party revealed a weak point or breaks their armor. Another use of it is if the big bad is fighting the party and they are completely stomping them then it might make things more dramatic to give the villain more health so they last longer or an attack they didn’t originally have that allows them to deal real damage to the party. Hiding rolls comes with a stigma though. If you legitimately have a good roll a party member might accuse you of fudging your rolls to make the enemy overpowered when really, it’s just a set of good rolls. It can make the party distrust the narrator but with the right party it makes for a more engaging narrative tool.
Which one you should go with depends on your style and your party. Operating in the open is usually for groups more focused on the mechanics of the game and combat than they are story. For those who are more focused on the story and characters, where the mechanics and gameplay are only there to help facilitate the story rather than be the focus, then a narrator who operates in the shadows and focuses more on making compelling events. But ultimately the right choice is the one that suits the group most.