I mean, if you roll one number and claim you rolled another, you’re lying. Now, there are plenty of tables out there who are quite happy with the knowledge that their DM may narrate results that are contrary to the dice rolls when they feel it results in a more narratively satisfying experience. But I do think it’s a better policy to be up-front about it if that’s what you’re doing. Roll the dice in the open, and if the results are something you think should be fudged, say so to the players. “Ok, if that monster crits right now it will definitely kill you outright, and I don’t think that would be a very satisfying end for your character. Everyone cool if we just call that a natural 19?” or, “look, this guy has maybe a turn or two left in him and he hasn’t hit once. What do you say we just call that a hit so the fight is at least some kind of challenge?”
If your players wouldn’t be ok with it if they knew you changed the results, then changing them without their knowledge is a breach of their trust. If you don’t feel comfortable doing it honestly, why would you do it at all?