There's no legal requirement, at least not in MA, but someone could argue it was "massprudent" to do so in the event of some issue, EG, in case you somehow or another got in trouble with the kopsch and they went full douche and wanted you to "explain" where all your guns went, but frankly if I reach that point I'm not talking to them about any of that shit, they can talk to whoever my attorney is (who is politely going to tell them to f*** off, or is going to ask me for that information in whatever way I can supply it to him etc. ).
I am a fan of the "keep records, but treat them like a poker hand - nobody gets to see that shit unless your lawyer tells you to play that
hand". This means doing stuff like storing them offsite, etc. Maybe scan and encrypt the docs and store the device somewhere offsite.
I've seen more than one case where "gun records lying around invited police fishing expedition, news at 11" stuff to make me not want to leave that shit
lying around in the same places the guns are. Some bad/shitty cops see gun records and they instantly get a prosecutorial boner and start getting delusions of grandeur in their head reading names and so on. Don't give them any free ideas. (like ole drippy says in ATHF, "Idle hands spend time at the genitals. " ) The only exception to this I would make is a photocopy of an NFA tax stamp paperwork for SBR, MG, etc, in states where someone is legally compelled to show this stuff to law enforcement and/or states that have a "illegal but not if NFA registered" clause in their laws.
ETA: Obviously if someone is an FFL there are a whole other set of record-keeping requirements etc laid forth by BATFE but that's not the kind of stuff I'm talking
about.
-Mike