I usually say that a ward doesn't need a threshold but it does need a boundary. All of those things that Dresden talks about that we define as cutting off this thing from that thing. A fence, a river, a doorway, a street, etc.
That's just kinda my feeling on the matter though, I'm pretty sure RAW mentions a threshold. Keep in mind though that a threshold of 0 is still a threshold (so businesses, abandoned buildings, etc should be ok).