I don't fudge dice and roll attacks and whatnot out in the open. On the other hand I do design the encounters, make decisions for the monsters, decide what tactics I'm going to use. I recently ran a game where some heavy-hitting giants could have taken out a PC but instead of focusing fire they split up their attacks. Of course they also had a low intelligence (hill giant sergeants), so they were just smashing whatever caught their eye.
But people should do what makes sense to them, what they feel will make the game most enjoyable for the group.
I have no problem with a DM that doesn't necessarily play their creatures with 100% tactical perfection. Some creatures are clever, others aren't. Some are clever but cowardly, or dangerous but foolhardy, or whatever. Personality is already going to be a big vague X factor anyway.
But I really do very much prefer DMs that play by the rules they claim to play by. For me, both the "game" part and the "roleplaying" part are essential to "roleplaying game." A game where the combined referee-opposing coach is constantly rewriting the rules, hiding this revision, and denying players the ability to learn from that revision, is a DM who has denied their players the ability to game. I can get freeform RP anywhere, so an RPG with no G is not really appealing to me. Likewise, I can get focused numerical gameplay anywhere, and usually to a much higher design standard than all but the very best-written RPGs out there--and without any of the scheduling hassles that TTRPGs entail.
If I'm playing a TTRPG, it's because I want to roleplay by gaming, and I want to game by roleplaying. That's why fudging (and its mathematical equivalent, secretly altering stats or HP values
after they're already in play) is such a problem for me. I would 100% always prefer the DM levelling with the players--or, for folks who find that too painfully immersion-breaking,
at least using diegetic means to achieve their ends, rather than deceiving me and my fellow players.
And a simple reason why I essentially always roll in the open: I'm just not that good at deception. My players often see through my work ages before I expected, and I'm sweating bullets just trying to not give it all away and thus spoil the experience. I just don't have the personality or constitution to keep up a facade like that forever.