Should the ionizer on an air-purifer be kept on all the time?

I have 4 ionizers/air-purifiers in 4 different rooms. Its a small house, but i have 3 dogs and a small child with one on the way. The house is also old, so it always seems to be dusty. Would it be bad to leave the ionizers on all the time? It says in the directions not too, but my house isnt the standard... any suggestions?