Methadone, and ALL opiates for that matter, restricts the production of saliva which protects your teeth from plaque, which in turn leads to gum disease and tooth decay.
And, yes, lifestyle (poor diet, heavy smoking, blowing guys in back alleys, etc) contributes to bad dental hygiene and general poor health, but the FACT remains that the continuous use of any drug that causes dry mouth will eventually lead to tooth decay.