For most firearms, all registering does is protect you.
Since there are a half dozen ways for the gun to lawfully no longer be possessed by you but still have you listed as its owner on the registry, the registry is meaningless.
I've personally worked with people who were in this situation in the case of a protective order. The cops ask about the gun. The gun owner says it was sold out of state and shipped to a dealer. The cops ask what dealer. If you were hiding something you could say you don't remember since there is no record keeping requirement. At that point you are lying and breaking the law. But its less risky than building a receiver into a firearm and then refusing to register it.
Unless you just want to burry it in your back yard. If you actually want to take it out and use it. Then not registering it is more risky.
Remember, if they ever actually do ban mere possession of these rifles, you can always just move it out of state and be in compliance. Just like with bump stocks.
Now, I'm not saying that if you were ever in that situation you should register all firearms. My inclination if I wanted to have an off-the books gun would be to do it legally. Keep the receiver as only a receiver. Then there is no legal requirement to register it. And if sh1t goes sideways, you can always build it with parts kept on hand.
The lowest risk action is always to do what you can that is 100% legal and off the books. Even if it means spending more money.
I know CT people who bought a number of pre94-ban ARs or lowers or full rifles on the secondary market prior to Newtown. At that time there was ZERO record keeping requirements. So no DPS3 was filed with the state. There also wasn't a requirment to register post-newtown pre94 guns.
So these people now have 100% legal guns that nobody knows about. Sure it cost them money. But if you have kids and a mortgage, etc. Its worth spending the extra to have a firearm that is both legal and invisible.