Actually there are no statutes in the state of Washington which would tell you that you can not open carry a loaded long arm. There, however, statutes that prohibit the concealed carrying of a long arm. If you have a holster for your long arm, then you can definitely open carry it.. (Not that I would, but that's just my opinion)
I've seen numerous people in the general vicinity of the area who open carry their pistols, and I've never seen any of the authorities give them flak for it.
On the contrary, I've read numerous stories on here about how they have been detained and harassed for open carrying their pistol. Infact, some of the stories are so disturbing, that it almost makes a person not want to even bother with it. Although, the more people open carry, the more the authorities will get use to it. Unfortunately, there are a lot of rookie officers who aren't aware that open carry is legal, and often harass citizens who do open carry. Fortunately, many police departments are posting training bulletins for their officers to read regarding the legalities of open carry.
Over all, Washington is a pretty good state when it comes to the open carrying of firearms. Western Washington seems to be a bit better about it than Eastern Washington (Judging from stories I've read)