I've been known to ignore dice rolls in demo games, but they are not proper games.
More occasionally we have ignored a single roll that would end a battle early or that would have an instant disastrous outcome on the outcome of a game - such as an Age of Reason battle end on double six before it has really begun. When this has happened the game has counted as having two results, the early end, and then the 'what if that had not happened' end. It is not very satisfactory, but sometimes, when the guys have been looking forward to a game all week, have travelled to play, and some poor schmuck has spent hours setting the game up, it is intolerable for a game to end mid evening.
As you say, rolls should never be changed in campaign battles. One disaster wouldn't normally spoil a campaign and might, perversely, add to it in the long run.