In tabletop I do it on and off as the story needs or to preserve the 'players feel like heroes' aspects if it won't compromise the rest of the story for anyone else.
As a ST on a MU. I think adhering to the rules more or less as written in the books or listed as interpreted by the MU in source - be it in help files, on a wiki, etc - is the better alternative because it saves you cries or accusations of cheating. Now. If I were inclined to fudge a roll be it to someone's help or my detriment - I'll be quite up front about it. Let the room know - and maybe tell them why. If for example I learn mid-combat my statted NPC is a bit more powerful than I anticipated for the people - and he hits someone with a death blow, I will let them know. Or give someone a non-standard roll or something.
I think it all boils down to the players and perspective though. Just be up front about it, I believe, and that saves you most of the corollary problems from rules lawyers and people who are quick to assume things.