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.
Source: DnD 5e DMG page 235 and 237
Funny, my Hackmaster book p. 113 says fudging dice is cheating. But you are free to roll dice with the devil.
“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.