Do whatever you want but the Dungeon Master's guide encourages DM's to (sparingly) fudge rolls to avoid your players getting screwed over by bad luck. It's not against the rules at all.
"Finally, this rule absolutely eliminates the
need for anyone, be he player or, so help me gods,
GameMaster, to fudge a roll. Fudging, also known as
CHEATING has no place in a game that already has
a mechanic designed to eliminate freak occurrences."
I guess you are right, DMs can fudge all they want. GMs keep their honor and don't roll dice with Satan.