I have a friend who left home at 18 to join the Marines. He used his parents' address for mail. He then worked for various overseas companies (doing oil related things) and continued to use his parents' address for mail, tax forms, and employer records. Now he lives in the US again, but in a different state than his parents. He says that since he moves every year or two, his parents' address is his "state of legal residence" and he uses it on tax forms, for insurance (he works remotely now), etc.
I'm guessing after a year or so de-facto state of legal residence changes (even though he has a history of moving often), and he should use his physical address on tax forms, insurance, and everything else. Is he right, or am I right?