As far as combat encounters go I always reserve the right to have a hit drop a bad guy dead at anytime. If the fight is going poorly the monsters suddenly have less vitality. The good thing is the PC don't know how much health the monsters have.
Fudging is fun. I would be concerned, here, about consistency. The next time the party runs into the same monsters in your campaign, they might be foolhardy. You may end up having to use your "right" as an obligation more and more often. And that means they won't ever really be fighting the full monster, so to speak. There's a bit of advice in the DM guide, which I heartily agree with, that it's better to give an easier challenge that the party can beat than one that they need a deus ex machina to have a hope at. (Unless, of course, you're using the challenge to deliberately force them to back off, like the guards at the gate.
