From what I've been told the whole house humidifiers will only get you
30-40% humidity, so you would still probably want a room humidifier near
your plants if you want higher levels than that. The whole house humidifier
can definitely help though. Like Joanna said, if you have run-of-the-mill
Phalaenopsis, Dendrobium, or Cattleya type orchids a few days of lower
humidity shouldn't cause any damage.
-danny