It's a myth. I have met hundreds, maybe thousands of long term addicts of meth, crack, and heroin... Lifestyle addicts suffer from massive tooth decay & dental problems, in similar proportions, regardless of which drug they were using.
Whatever the pharmacological impact of meth usage on oral health, it's pretty clear to me that it's a much smaller factor than the general gone-to-hell life of a full time addict.
Combine that with also constricting the blood vessels of your gums, and you get a great mix of infection of dying tissue.