I have so much stuff spread out all over the table and am hopping up and down so much trying to keep track of things, I couldn't possibly hide a die roll; nor do I have the grasp of rules and mechanics necessary to cheat in front of my two game-mechanic players; nor can Itrack of the status of each PC well enough to know when to cheat to save them. They beat the tar out of my bad guys fair and square.
I think I sometimes "cheat" by accident in the heat of the moment - forget to use a skill, for example. And I once ranged them against archers who did not hit once during the entire encounter, and I wondered later if I hadn't grabbed a couple of old-fashioned d-20s, numbered 1-0, 1-0, that the coloring had worn off of. It was too late to investigate, and since the PCs had organized "Born in East L.A." rush of the wall by civilians to cover their own escape from the walled city, I had the logical excuse that these were guards who couldn't get into recreating the Boston Massacre or Kent State. Anyway, that's different.
Since I want my PCs to survive, I give them ready access to healing magic, pace adventures so that they have a chance to swallow a potion or talk to the priest before the next combat encounter, allow my bad guys to make in-character bad judgements when things look really dire and nothing huge is at stake, am not rigid about in-party cross-chatter, and allow the players to at least try out any wild-hair desperate heroics they can come up with. Good PCs will keep each other alive if you let 'em.