I have my own opinions on the whole con being in Vancouver thing but there is no point talking about them, since it doesn't matter. The decision has been made, there won't be any changing it. There is definitely a difference though in persona's and attitudes of those that attend Kumoricon and Sakuracon than those that attend Game, Sci-Fi and other larger anime cons though. Not necessarily a bad thing... but it is definitely different when looking at it from a professional con-goer experience.
Here we go. It appears the rumor you mentioned is not true, businesses are not required to offer exemptions to out-of-staters, only permitted to do so with certain requirements. And of course, the law is very complex.
Just to be a bit more specific about the law, it really isn't that complex really.
The exemption is limited to purchases of items that are taken and used out of the state. Purchases of meals, hotel rooms, laundry services or other services or products the nonresident uses while in Washington do not qualify for the exemption.In short meals, food, hotel rooms, services, etc do are not required to give you an exemption. You can still ask if they honor them, some businesses do for hopes of repeat business. Of course places like McD's, BK, Taco Bell, etc usually won't though.
If you are purchasing clothes, dvds, electroncics, actual physical items then they are required to give you the exemption.
Please remember though they will require a proof of Oregon residence and it usually has to be a Oregon Drivers License or State Issued ID. A school ID, passport, even a letter with your address would not count as valid proof of residence.