We brush the teeth of all 8, daily. If you use an enzymatic toothpaste, it does a much better job than just chews can. In our experience, anyway...
I think a lot of it is genetics--I've know dogs that never had their teeth brushed and rarely got anything to chew and still had gorgeous pearly whites their whole life. Our dogs' tooth-related genes aren't as spectacular as that.
So we brush.
Does a pretty good job.