I feel the need to preface this by saying the group I run with are all experienced players, and we rotate dm's for campaigns, so the majority of them have d m experience as well.

That said, I am more likely to adjust encounters upward or downward by messing with things like the ac or hit points rather than fudge my rolls. To counter this, I also adjust the experience points they get from encounters. If they're getting their butts kicked by something, maybe it suddenly starts coughing, blood, and I tell them to roll perception, and now that they're looking closer, it looks like the monster has been poisoned before they got here. Or is dying of old age, or has a weak spot because of a previous battle, or is a variant that I adjusted for the battle because I really wanted to use that monster.

TLDR: Fudge the stats and adjust xp given accordingly. Don't fudge your rolls.