I'll cheat if the players are getting an incredibly bad (or sometimes good) run of luck. Basically, if through no fault of their own, they're getting hammered (They keep rollings 1s, I roll three crits in a row), are adjusting their plans to account for that, and are still getting beat up, I'll give them a break. Likewise, if an important enemy is about to get killed in the first round because three players crit on him after he loses initiative, I might fudge a number or two.
However, if the players are in trouble through their own bad decisions, that's their own fault, and its up to the dice to save them. Likewise, if they get the drop on an enemy through really clever planning as well as some luck, that NPC is in trouble... which he should be.
Basically, I'll cheat a bit to compensate for really abnormal rolling, but never, ever for player decisions.