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.
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.
