Things I do not do:
Adjust dice rolls. All dice are in the open so the players can see what I rolled.
Adjust stats. If I wrote down the orc has a particular skill, weapon or spell, that stays.
Deus Ex Machina rescues. If there isn't a logical reason for someone to show up to help them and I didn't plan it in advance, then they are on their own.
Things I have no problem doing:
Adjudicating that the enemy retreats if it feels like they have had enough. ("As their leader falls, the orcs turn tail and leg it")
Declare a morale check if the above may apply and roll that in the open. ("okay since they are faced with fire, lets call it a 40% chance the wolves flee")
Declare something happens without rolling ("The first orc takes a shot that goes over your head. Do you guys draw weapons or do you wanna do something else?")
Declare a combat over when its pointless to continue ("The last couple of skeletons gets mopped up")
I dont know that these are super logically consistent or whatever. They are just how I prefer running the game and I've found that they tend to result in the group generally trusting in the world and the GM to be rational and predictable.
I don't prep in super detail, but I do try to stay faithful to what I do prep ....unless I forget something I had meant to put at a location