That's what I always thought, but I read this page on Behr, see the 8' Ceiling notes: http://www.behr.com/behrx/inspiration/practical_5.jsp

I guess a warm white then? Or whatever tone I use as trim (warm or cool)?

I don't agree with what they say. I've seen many low ceilings painted with colors, and it always looks bad.

My house is older and has all 8 foot ceilings and every room seems to have a different trim/ceiling white. As soon as we moved in, I went and chose a white that I will be using for all ceilings and trim as we go through and paint each room. It's a neutral white, neither warm nor cool, so it goes with any color in any room, and gives a sense of continuity from room to room.