Wow, I'm really rather stunned. I expected to be a tiny minority, and I see almost everything I wanted to say already said, and said well.
So put me in the camps of:
- Desire to fudge is a sign of a broken, misunderstood, or poorly selected ruleset--and better fixed by making changes thereof.
- Fudging dice bad; fudging other things isn't really "fudge". It's all DM fiat whether I adjust the encounter when I first write it, the night before, or one minute before the party sees it.
- However, it is the DM's job to make consistent decisions. That includes being consistent about how adjustments are made. One of the most important decisions is when to roll the dice.
- If you fudge dice rolls secretly, even the most foolish, clueless players will eventually suspect. This suspicion will damage the game far more than any other alternative you could select.
That said, like all metagaming influences, there are times and places. Two of the most critical:
- You (your table) otherwise like the ruleset just fine. You don't see an easy way to fix a niche problem, that could do some massive derailing if it arises, but isn't likely too. You can't stand ret-cons. Quietly fudging these situations out of existence is the easiest way for you to deal with it.
- And, this is all understood up front by everyone there. That is, the players don't necessarily know--and probably explicitly don't want to know--exactly when you deal with these situations. They have simply empowered you to do so, and that's that. They expect you to not fudge outside those parameters. This neatly nullifies all the bad effects of secret fudging while retaining the fast play/immersion enhancing aspects (for some people).
And one qualification: The dice roll itself is less critical than it what it means. The die is a tool. Say I have a wandering monster table with 8 results. I've decided (DM fiat, setting notes, whatever) that only the first seven apply in the current area. I roll. I get an 8. I reroll until I get something else. That isn't fudging. That's merely generating a number from 1-7 on a d8. OTOH, if I think the whole table is fine, roll, get the 8, and then decide the 8 didn't happen--fudge. To me, anyway. Not that you can't DM fiat to override those kind of decisions--it is borderline anyway, and out in the open or behind the screen is irrelevant here. But I think a big part of training yourself to be consistent is thinking about the possible results before you roll.