I may be a little late to the thread, but I think I can answer this question as I have one deaf child and the rest of the household is hearing (I have three children seven and under; my deaf son recently turned 5):
As a family, we are committed to learning ASL, so if it's information that is important for all the children to receive then we sign it so that they all have equal access. Sometimes we will go voice-off for a day outing -- to the zoo, for example -- for the practice, to include my deaf son as much as possible, and because it's fun (you should see the looks we get; I've noticed that hearing people suddenly stop talking when they think they're around Deaf people). My wife and I sign as much as possible when communicating with each other, though it is often sim-com because our signing skills are still developing. If there is information being conveyed in the room and nobody is signing at the moment, we make a judgment call and convey the information to my deaf son if we think it's something he should know or might want to know. For instance, my wife and I were discussing one of us running out to pick up a video. My hearing son overheard us and commented, so I took a moment to explain the conversation to my deaf son so that when one of us suddenly walked out the door, he wouldn't be confused and wonder what was going on. What's really precious is when we catch our kids in a room by themselves all signing away to each other.
In short, choosing to have a bilingual household is a pretty major lifestyle decision and takes commitment, discipline, and the support of the entire family.