I very rarely leave it down. If I want a "down" style, I usually curl it, or blow dry then flat-iron it (though this is rare). Even bumming around the house, my hair is never down; it is always up in a ponytail.
It is not often that I go two days in a row with my hair styled the same. I switch it up between braids, buns, twists, etc. Sometimes though, I want it down and wavy. Curling it (even with it brushed through) just tends to look too contrived. That's where "natural" waves come in. I reverse french braid slightly damp hair, then sleep on it. Boom.... naturally wavy hair the next day that is soft, bouncy and beautiful!
These waves are after a 9 hour day at work, and walking through snow to and from my car.... so they were a bit cuter this morning, but they held up really well!