I'm actually quite curious about this.
Firstly from a definitional point: If there are strict rules then (known to the players or not) it's not really fudging - it's just application of a rule ie Adjudication. For ex. If the rule is (completely arbitrarily just for ex.) every time a d20 rolls a 11-12, I treat it as a 13 - well ok. I don't think that's fudging if it's a strict rule and it's simple adjudication of that rule!
But second, what warrants strict rules on changing the result? Is it something like: If a creature with multiple attacks rolls a crit, I only count 1 of the attacks as a crit?
Genuinely curious.
I only fudge under two very specific circumstances and then only to achieve very specific results.
1. In events of extreme luck. With the following requirements. If any requirement is not present, I will not fudge.
A) Extreme bad luck on the part of the players. Not just bad luck, but extreme bad luck. Events where the players are not rolling above single digits and are failing all or almost every saving throw.
B) Extreme good luck on my part. I'm not only hitting out the wazoo, but critting a bunch of times. Making all or almost all of my saves.
C) No mistakes or bad decisions on the part of the players. Regardless of luck, if they shouldn't be in the position they are in and made poor choices to get there, nothing gets fudged.
D) The party is very likely to or definitely going to TPK over the extreme luck through no fault of their own.
If all of those are present, I will specifically fudge only enough to give the PCs a fighting chance. I'm not going to fudge so that they win or have an advantage, and I never ever fudge to help my side of things. Once they have a fighting chance, even if the extreme luck continues no more fudging will happen. The party could still TPK or end up with deaths.
2. If I screw up the encounter difficulty. In the event that a badly misjudge an encounter and it goes into deadly territory, I will fudge it down to merely a hard encounter. I'm not going to kill or TPK a group over something I misjudged.
That's it.