Just for transparency, I'm not a big fan of casually fudging stuff, and that's not what I'm supporting. I'm supporting the tactical usage of slightly (this word is very important) nudging values to maximize the player experience for little cost.
A good example would be the idea that you just had a Paladin crit, expend a high level slot to smite, and get the boss to something like 2 HP. The Fighter is next and the Fighter has Graze, so they will kill the boss with their first attack regardless of what else happens. Do you:
- Have the crit kill the boss and let the party celebrate the moment (and by extension celebrate the Paladin)?
- Make it go to the next turn and have the Fighter automatically kill the boss?
My personal stance is that 2 is unaligned with my desires and I'd be unsatisfied with the result regardless of if I was the Paladin, Fighter, or otherwise, though I'd be pretty miffed if I was the Fighter and especially miffed if I was the Paladin (assuming the DM is experienced enough to understand the dynamics I'm referring to.)