Trimming the door probably doesn't actually hurt the rating. The reason is that once you modify it, then it isn't the same door that was engineered and tested. Those rules are more in place so that someone doesn't go and cut a window into the middle off the door, and assume it still maintains code rating. Where I am located, this goes for the energy rating of doors too. The door is rated as an entire unit, and and once modified, loses it's rating. That being said, most fire doors, and energy rated doors are exterior/dividers that should be measured slack of the opening and hung within the frame that they come with(around here anyways), so trimming should not be needed.