Identifying when to throw those last few hit points in the trash and call the fight a “win” for the adventurers is part of what separates a great GM from a good GM.
Your games pacing, the spotlight moment for your heavy hitters, and respecting your players time and emotional investment in a combat scene, are all much more important than the big bads last 10 HP.
I guess, but in many systems this is on like a razor edge of GM fiat / “are you sure you don’t want to just write a book?” problems. I don’t really want the fight to go on until the GM unilaterally decides it’s been cool enough. There are games with more explicit rules and guidelines about it, though.
If there is still danger and drama left in the encounter I would agree with you that tossing out HP for “rule of cool” isn’t a that good of a practice. That mook with 1hp left can still make an impact.
But if things in essence is over, that the PCs are just cleaning up, then do go ahead and let those remaining hp slide. The outcome is already certain, why drag it out? Wrap it up and move on.
Related is to consider adding morale rules. That one hp mook who moments before had its friend cleaved in one swing may falter and surrender/run away. The result is the same as if the last hp was handwaved away but doesn’t feel as much of cheating. Also surrendered foes tend to drive the narrative forward much more than alive ones.
I definitely had some specific games and circumstances in mind. The core of the message is “feel free to do what is right for your game, don’t be bound by the rules.”