Besides teachers, I have heard lawyers, doctors, accountants scientists and politicians addressed as "sensei". I guess its a respectful way of addressing someone in these professions, a way of showing that you recognize their status in society.
Should it be used all the time? Is it necessary to call someone "sensei" everytime you see them? Can seem like blowing sunshine up a hakama.
Actually this is very common in Japanese society. Even company presidents or heads of associations are always addressed as "president" or "chairman". Some of my Japanese friends who were presidents of their own companies didn't get into this but others really got off on it.