In our group there is the understanding that the GM, by definition, cannot "fudge" sind he and his narrative *is* the rules.
Personally, as a GM I do roll a lot of things in secret simply because the players must not know how well they rolled. This is especially true for perception skill rolls, but also for many lore or even language skill rolls where ignorance is the key.
In combat, rolls are usually made open for all to see. Here, we have the understanding that, in our main campaign, a character shield mechanism applies by which the GM is expected to give all characters a fighting chance to avert death, to the point of altering crit results at a whim. We have found maiming to work much better than outright killing. It helps a bit to keep power creep down if the party has to use up resources and call in favors to save or heal a criticlly wounded comrade, and several characters have sufferend permanent injury resulting in things like reduction to their potential stats, permanent negative maneuver bonus, or receiving negative traits from Character Law.
We have played in alternate settings or campaigns where a strict "no character shields" rule was in place. It's also fun, but makes for a much different game with many more random deaths. It works if clearly communicated by the GM beforehand, and if the campaign doesn't rely on the same characters seeing it through.
