This is a bit of a touchy subject for some people (including a few I know personally), and for good reason.
What is it?
Before I get into the subject, let’s define it. Broadly speaking, fudging a roll means that the DM rolls one or more dice in secret, then reports the result as being different than it really is.
You might ask, “Why would a DM do this?”
There may be a few different reasons.
Some DMs are interested in keeping the party alive, some want to kill everyone. In either case, altering the results of the dice rolled can exacerbate the results desired.
Another reason might be to move the group towards a desired plot point, one which requires the group to either slay something above their weight limit, or requires them to be in desperate need of outside help or healing.
There may be a lot of other reasons, but these serve well as examples.
There’s another form of fudging rolls, which is altering the DC of a saving throw in order to achieve a particular result.
For example (and of course it’s just an example!), lets say that a party member is gored by a wereboar and needs to make a CON save (DC 12 in The Monster Manual), and rolls a 12 (which makes it by the way), but the DM decides that they really want to mess with this character so they raise the DC to 13 (I might have to share the full story of this later, but it’s not just a random example).
Ultimately we can divide everything into two categories: Benign and Adversarial.
Benign: Should I Do It?
Obviously this is going to be a personal decision no matter what way you approach it. A DM (presumably) has their own reasons for running a game the way that they do. However, my thoughts are as follows:
D&D (or whatever tabletop RPG you’re into) mixes role playing, strategy, and chance together, in an attempt to imitate life in a fictional setting. A major part of that is brought to life through the rolling of dice.
If you remove the chance element, or significantly reduce the risk, you also remove the excitement that goes with it. The game becomes tame, safe, and ultimately, probably boring.
There are times when I have fudged a number a bit, normally to prevent a TPK from occurring. I don’t mind TPKs in a major boss fight, but sometimes the party just rolls really bad consistently, or the DM runs an encounter that is just a bit above the punching weight of the group and needs to balance it a little bit. Remember, as the DM, you’re still prone to mistakes, and fudging dice in favour of the group may be a way to counter some of those errors.
It may also be a way to speed up an encounter from time to time. If a party member hits a foe and leaves it with 1 hp, if I’m trying to end the combat, the creature will take the extra 1 point of damage.
But seriously, don’t make it a regular habit.
Adversarial: Should I Do It?
As a rule I am strongly opposed to adversarial DMing, but occasionally there’s a case to be made for it.
Sometimes the group is extremely well balanced and needs something to liven them up, or you’re consistently rolling misses against everyone and there’s no challenge at all. There may be a case for fudging a roll so that someone (seriously, at least once in the encounter) gets hit. I don’t think I’ve ever actually done this, but I have been tempted.
You may be running a game where a more adversarial style is fitting, but in that case I would strongly advocate against fudging rolls. After all, you probably don’t let your players fudge rolls, so it’s kind of cheating if you’re taking the approach that “It’s me against them.”
At the End of the Day…
If you’re going to fudge dice rolls, don’t do it often. For the most part I tend to roll in front of my players. The main exception to this is when I’m making rolls for creatures that they aren’t aware of yet (most commonly stealth checks).
For my part, I like a game atmosphere where my players feel like they can trust me, and where we’re playing together, not against each other.
If you occasionally feel like a small tweak will make the game more fun and interesting, then that choice sits before you.
At the end of the day, it’s a game. If you’re going to cheat, do it for everyone, not just yourself.