The only reason to slope a steam shower ceiling is to prevent condensed water from dripping on your head while you're relaxing. You may not need to slope the ceiling to accommodate the steam.
Why Not to Slope Ceiling
It can take the better part of an hour before condensed water in a steam shower becomes heavy enough to start dripping. That's longer than most people will sit in their steam shower.
The thought is that a sloped ceiling will prevent water from dripping. That may be true if the ceiling rises 6-inches for every horizontal foot, but a rise of 1 or 2-inches per foot isn't going to stop water from dripping.