To Slope is the answer! The main reason to slope your ceiling is to prevent accumulating moisture from dripping down on you while you bathe.
Moisture accumulates on all surfaces and then gravity takes over. This moisture easily runs down your walls but not so with the ceiling. The moisture will either run down a sloped ceiling to the wall and then down the wall or it may drip straight down like rain taking the path of least resistance.
The surest way to avoid dripping is to install a sloped, smooth, solid surface with no seams. Tile edges, grout lines, and raised surfaces of your ceiling all create a roadblock, if you will, preventing the condensation from reaching the wall. This roadblock becomes the easiest path for the condensation to drip from.
Read more about Ceiling Height & Slope