It looks like to me you got all the advice you need on whether to register or not. But a quick tip in case you or others do put the registration
numbers on your boat: You have to leave a space between the letters and the numbers.
i.e. WA 984222 IX, NOT: WA984222IX. I know this sounds incredibly petty, but I had a full boat inspection on the water by the Coast Guard last year while fishing near Neah Bay. They actually showed me the state regs requiring the space, then wrote me a warning ticket for it. I was just a random inspection that day and everything else in my boat was perfect, plus I was fully cooperative so they had no reason to get nit-picky. To this day I can't believe they would be that petty. They even told me they keep a record of the warning citation & should I be checked again and it wasn't corrected I would get a full citation! I was glad the CG was out there working to ensure mine & others safety, but it sure seems they have better things to enforce than registration spaces.