Your choice.
I prefer to use prudence in randomness. But it would be more disheartening to my players when they finally meet Mr. Baddie if the encounter played out like he was a wimp and/or buffoon. Of course, I would never cheat my players if they do something clever or such. But, I disagree, a BOSS is suppose to be climatic! Otherwise, they are just another encounter...
Also, I won't penalize a player, either, sometimes making a critical hit that would kill them into a normal hit (unless I know they have a way to restore them later on) or decreasing the effect of a spell or something on a failed save. Luck is part of the game, to be certain, but if you are such a slave to fate as to not use good judgment, then you are doing a disservice to your players IMO. But, that is your game and you know your players best.
For the record, everything I wrote was "you can", not "you should". I have most of my encounters go as the dice indicate, but the OP was about what happens when your finale fizzles because of a single bad die roll? My point was, you have the power as DM to say it won't happen and give your players the enjoyable and challenging reward of a good, dramatic encounter.
And the point of having stats? Well, they are the guideline. Sometimes the baddie has too many HP and is about to TPK. Why not let the last hit, when only one player is standing, win out the day and send the foe to the floor in defeat? That makes the encounter heroic and something to remember, instead of "Hey, remember when the monster killed us all off? Wasn't that a great way to end the campaign?"