Fudging is very bad. Your players might never know you did it, but it many cases it robs them of their agency. They make some choices, you have to make some rolls and you just outright decide the outcome. It's very unfair and there's very few players I've DMed for that would have appreciated it being done to them.
The counter-argument is often something along the lines of "I'll fudge a roll to move things in a direction more interesting for the story". I disagree with this. The DM doesn't get to decide what's the story and where it goes. We already have a ton of agency over what happens, when the dices are cast, we must respect the outcome. If we don't, then we only do when it suits us, and then why roll dices at all? In my opinion, TTRPGs are not about crafting a story but discovering a story together. The players bring their characters, possibly some backstory and their decisions. The DM brings some content to explore, a situation and some reactions and what happens between these two things is the emergent things that's precious to these games. Fudging is one of the many ways where you make that spark wane.
The only times I've fudged a roll is when I took a bad call, and as I'm making the roll I'm like (this was a bad idea, it adds nothing). We often make split-decisions. Something that sounds awesome like "I'll make a roll to see if the ancient staircase crumbles under their feets" can quickly turn to "What happens if I fail this roll? Do they have any way of not falling? Do they have any agency? Does it further what's happening? Why am I rolling for this?". In these rare moment I might pull a quick "Oh, how lucky, it holds until you cross" and then spray myself with water for being a bad DM.
But that happens rarely. Once or twice a year, maybe.
