I would think that you would want to care for all of your horses hygiene, teeth included. I'm sure if teeth care has been neglected it would be harder to take care of (more work needed for what was lacking), but I think if you can take care of your horse proper and not miss any-times to check up on the teeth, all should be fine.
From an
article I found online recently: "A 1994 survey estimated that 80% of horses have dental problems."
Could you imagine what that percentage is now in 2007? Here is the article if you want to give it a quick look over, I found it more than interesting, and sadly as much as I would love all pet/animal owners to take care of their responsibilities, not as many do, so I would estimate that % is about the same now, if not more.
