I don't know. And I don't say that lightly. I have read a number of books on Hitler, as well as the Third Reich more generally, and can not recall any mention of his flossing habits, or really his oral hygiene in general. All the books are digitized, so a OCR search was not successful in finding anything that escaped my memory. The most I can say with certainty is that he visited the dentist and had bridge work done at some point, as that is how his dentist identified the piece of jawbone recovered in Berlin by the Soviets. Beyond that, I can't provide you an answer one way or the other, and while there may be a book out there that mentioned it, none of the (considerable number of) ones I have do.