Here's a thought:
What if you wrote on a sheet of paper:
10 Goblins, 11 HP
And then as each goblin took damage you counted up, when a goblin was at or near 11 damage taken you declare him dead. Some might die at 9 damage, some might die at 14.
In other words, is the OP question relevant if you as DM don't know the monster HP, only the "average", and played accordingly.
I stopped doing so many years ago, because my players now realize that, while I may strive for a mix of encounter threats, once they commit, it's fixed in stone, and success or failure, it depends upon them, their actions, and the dice. This awareness heightens their sense of risk, and makes for a more exciting game.I can't change HP during combat anymore, not since I started tracking it in view of the players a few months ago.
I would actually find that the least offensive way of doing it if a DM insisted on doing it. I would, however, want to be informed that the DM runs his monsters that way so we are on the same page.
Why not have the dragon die from the attack the barbarian did? It's an epic moment, and from a story telling point of view, that is when the dragon should have died in my opinion. The remaining 1 hp is irrelevant. I think a DM should give his players their well earned victory, and occasionally fudge the numbers a little.
Look at it this way: Suppose the party is fighting a dragon, and the barbarian rolls a critical, and does a huge amount of damage to the beast. He describes how he leaps from a nearby rock and plunges the sword down into the neck of the beast.... blood is spraying everywhere.... but statistically the dragon still has 1 hp left.
The next turn, someone throws a rock at the dragon it hits and deals 1 damage, thus slaying the dragon.
Why not have the dragon die from the attack the barbarian did? It's an epic moment, and from a story telling point of view, that is when the dragon should have died in my opinion. The remaining 1 hp is irrelevant. I think a DM should give his players their well earned victory, and occasionally fudge the numbers a little.
Because I'm playing a tactical game of problem solving and resource management, and the epic moments have value only if they emerge naturally.
The rules are only there as guidelines, in my opinion. They should never get in the way of telling a good story.
So the topic of this thread, to me, is not really about hard rules vs guidelines. It's more a matter of transparence.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.