From what I understand, unless you eat a 100% "clean" diet, you will ingest enough B12 from fortified foods like nutmilks, nutritional yeast, etc. I don't see any issue with taking a supplement though, your body will just eliminate the excess. I know that I have to pay way less attention to my diet and general health than the non-vegans around me, but perhaps that is just a quirk. They all take multiple vitamin pills each day and weigh portions/count calories/restrict intake, whereas I don't need to. In fact, it can be difficult to eat enough during the day. I think it helps that I work outside and try to stay pretty active.