I think my ultimate point here is that I see this as an arbitrary dividing line.
A lot of lines are arbitrary. But sometimes there's value in having a line and saying, "I will not cross." For instance if I set myself the task of working on a project from 8pm-10pm Mon, Tue and Thu, I could've made it a different time and/or day, but there's value in setting a rule and sticking to it.
The other issue is that fudging a dice roll is 'cheating', deceiving the players, in a way that having an extra couple of monsters join the fight from an adjoining room isn't. Well, that is also cheating, kind of, but it's not such bad cheating. One could make a sliding scale, similar to what Benimoto has done. For me adjusting a dice roll is the worst and I won't do it. I don't think I'd ever change a monster's stats in the middle of a fight, but I have adjusted a monster's powers at the start of a battle (I did that at least once last campaign) where I looked at it and thought I'd just made it way too powerful. Fairly often I've planned encounters without defining the numbers of enemies present and made it up on the spot, which some GMs would probably regard as bad form.
I appreciate what you say about these lines being arbitrary. They are, in a way. But our lives are full of rules that are at least somewhat arbitrary.
All that said, I fully respect that you have chosen to fudge. Every GM is different, every table is different. There are no universal rules of roleplaying. (Well, except that Mary Sue NPCs are bad

)
I've played with literally hundreds of different people, must be dozens of different GMs. There are so many different viable styles. I once played in a game which was very railroaded, and it worked extremely well, even though I'd normally say railroads are bad. There was one particular scene where the BBEG was clearly predestined to escape, and my PC was trying to stop him, and I thought it ended up being f--king brilliant with my character being pushed to the absolute limit (and beyond). The GM was coming up with really creative ideas to explain how the villain got away (on the back of his mutant shapeshifting dragon). It turns out mutant shapeshifting dragons can be surprisingly hard to stop.
And yet we are told that scenes where the PCs cannot win are always bad, that railroads are always bad. They're not always bad. They don't work for some groups, but they do work for others.
The moral is - there are no rules. Do whatever works.