While an FtM will never be forced to serve under a draft call up [you're medically unfit under the current standards, and therefore considering 5A- ineligable], you do need to register to avoid the aforementioned issues of not being able to obtain federal benefits from various sources.
I was not aware you were automatically registered when you changed your gender with SS. That's good to know.
However, I'll be 27 [at least] before my gender is legally male. I guess I'll cross my fingers that 8 years of military service is sufficient as to explain my lack of having registered for the draft?