We designers say it all the time because its true: the ceiling is your 5th wall. It's as much an opportunity to reinforce a design statement as the other 4. Michelangelo dedicated 5 years to the ceiling of the Sistine Chapel. He made his point.
The ceiling is a blank canvas, so be creative. Ive heard people say " paint the ceiling white to make it look higher". Nothing really makes a low ceiling look higher, but a dark color can emphasize a low ceiling, which can be dramatic and intimate. So if you've got a low ceiling in a room that lends itself, turn an architectural liability into an asset but painting it dark.
I love this example of using color on a ceiling to reinforce a theme. In this sunroom, the ceiling is painted a sunny blue in order to blur the lines between inside and out. The decor remains neutral and the blue sky and plants do the talking, making this inside space feel very much a part of its outdoor surroundings.
In this dining room above, the client wanted a formal, traditional style but to give it an element of freshness and to avoid it looking staid, we painted the ceiling a rich dark blue which also helped to define the space in this very open floor plan. With all the beautiful elements in this room, a simple coat of paint on the ceiling somehow steals the show.
This example above is one where the painted ceiling IS the design element. The furnishings, floor covering, even the throw pillows are kept quiet in order to reinforce what is created on the ceiling. I believe this is an example of troupe l'oeil which literally translates to "fool the eye". The ceiling is perfectly flat, but the paintwork creates the illusion of tenting or a rising ceiling. Beautifully executed.
Lastly, this image is a brilliant example of stenciling on a flat ceiling. In this instance, a stencil was used to create the impact of a ceiling medallion where there was none. It doesn't get much more elegant or impactful than this and stenciling is not difficult to do - its just time consuming. But with a little patience, you will be shocked at what you yourself can produce.
Use your 5th wall. The above are a few examples of what's possible, but there are no boundaries. So be ambitious and enjoy the process, knowing that there are very few things that a fresh coat of primer can't erase :-)