I make it a point to NEVER EVER alter the dice rolls or cheat for or against the player characters, EVER. Why even bother to play the game if you are not going to actually play the game? This rule is violated by only the most horrible of DM's!
This is a WELL known fact at my game table and something all new players are made aware.
There is one minor point of concession I am willing to make though. When rolling characters the point by system while adequate in most regards...ok, my players gone? See I cheat like I'm card shark in Vegas Baby! I fudge dice rolls(mostly by having 20 siders already behind my screen and if I need to cheat I just roll a die and act like the die I rolled is one of those that have been waiting behind my screen. I subtract hit point/add hit points lower and raise saves(I have a list of made up modifiers and reasons to choose from if it ever comes up) I write everything down as proof but I write so badly no one can ever really read what I wrote! I subtract whole encounters and make them up on the fly all to just keep my game fun. I HATE to kill player characters off unless they are really asking for it, it spoils the fun so I just don't. I take them hostage, enslave them, feed them to some monster that is not yet hungry ect.. Lucky for me I have two players who get tired of playing the same character pretty often and once they start just asking for a good death by being foolhardy to the extreme I just arrange for the bloodiest nastiest pc death I can think of. If I can make it seem just unfair bum luck dice rolls while doing that...all the better. I will suddenly make the stable boy the real murderer and the man who threatened them a innocent who was set up on the turn of a dime. Whatever makes the game more fun. My players really want this as well. When I try running a game and just let the dice fall where they fall the players quickly get upset. See they HATE CHEATING in ANY FORM as long as you know, they WIN. They want to beat the odds and win because they are awesome! When what actually happens most of the time is they do well for a bit and then roll sucky and die ending their fun. The few times for various reasons that we played like that, they had many issues with those games. So what they really want is the best of both worlds! They don't know about the cheating and so are AWESOME!
So besides that one little aspect of my game, no fudging EVER! THAT"S JUST NOT COOL!