Methadone causes dry mouth and tends to cause a craving for sweets and weight gain. If you're living a healthy lifestyle, eating right, staying hydrated, and taking care of your oral hygiene, your teeth will be fine. Then again, the same could be said for almost any substance, including crystal meth.
The problem is, these drugs can cause behavioral changes that can contribute to you neglecting yourself. Not for everyone, but for many, especially tweakers since meth changes your behavior so radically. If you've spent time around tweakers, its easy to see the damage they've done to themselves. Likewise, spend time at a methadone clinic and you'll notice many of them have the same types of problems - overweight, bad teeth, etc. Of course there are some that stay healthy, but it's certainly a minority, judging by the 3 years I spent at the clinic. It was a very, very depressing place.