My ID recommended a 1/2 tint of the wall color, rather than white, to showcase the trimwork. Our wall colors are fairly light to begin with, so the ceilings are light as well...they just provide a tiny bit of contrast with the trim.

My trim is SW Creamy in semi-gloss. My ceilings are SW Creamy in flat.

But, I like Nini's ID's idea, and have clipped her post for future reference (if your walls are fairly neutral).

50% of the wall color for the rooms that are off white and beige. 30% of wall color for the rooms that have colored walls.