I fudge sometimes. I generally only fudge in favor of the player or of the story. Mages make the latter difficult, but that's just because it takes a lot more work to create an intriguing and difficult mystery for them.
I had Theno make "rolling blind" possible on Eldritch both to keep the mystery, and also because of this, somewhat.
However, if your character does something monumentally dangerous, that is obviously so, with prior warning, and they suffer for it--no fudging. If a character dies outright, I don't tend to fudge rolls; I may fudge time constraints, etc., if they are, say, "bleeding out" or whatever.
One thing that CofD has that makes this super easy is Conditions, basically because if I have to fudge a roll, I can, and then I can give the player who's (allegedly) getting the shaft a beneficial Condition.
Sure, your perception roll would have ruined my scene so I fudged it so you wouldn't see something--but now you have the Attentive Condition, which you can resolve next time you're going to be ambushed to avoid the effects of being surprised (i.e. you'rfe so attent to your surroundings you apply your Defense against a succesful ambush attempt) You also gain a beat when you Resolve it.
So yeah, I fudged--but you got something out of it.