No requirement to register currently owned guns, although pistol sales via FFL's in WA are already registered with DOL and State Patrol! Don't worry about guns currently owned that were purchased through private sales. In theory, guns you posses that are manufactured after early December 2014 will legally need to show a "paper trail" to you unless gifted. Although illegal and not advocated, one could continue to buy and sell older firearms without doing the background check and there would be no a way to prove when the gun was traded!
I don't believe law enforcement will actively do firearms checks on every weapon they see. If however the gun is part of an investigation and is new production, you better have done the transfer or you could be prosecuted as a by-product of another crime.
If you are concerned about registration or paperwork, you have 1 month of private sale exemptions left!