I don't like the idea of modifying hit dice during an encounter. It reminds me of elementary school gym class where the teacher would constantly change the parameters of the game to keep a 1-2 point spread. Blow-outs are part of games and, as a former athlete, I'll say that I absolutely love blow-outs. Going up against a feared rival and completely dominating was satisfying. Blow-outs are only a bummer when either a) you lose, or b) you have no personal stake in the game. (Like most super bowls I watch.) As a GM, I almost always lose, so I just have to embrace it, and if my players get mopped up, well, that happens rarely enough that I'll just call it a growing experience.
I imagine my sessions would be easier to run if I kept my finger on the scales a bit more — it would let me keep the pacing on track with whatever I'd planned before-hand and keep things a little more predictable for me. But I'd like to force myself to look for more innovative ways to up the ante or keep up with my wily players.
I will fudge rolls or hp when a combat is clearly over and there is no believable way for opponents to surrender or flee. For instance, I have fudged zombie constitution saves to keep those bastards from getting up. Again. And again. And again. (The player characters were in between the zombies and the only exit. Zombies don't surrender. Honestly, it's a big downfall of undead encounters — undead just don't give you a lot of options beyond braiiiinzzzz --).