I don't know about LEGALLY required to... its one of those things that's becoming less and less known to Washintonians and the business owners. Vancouver is still pretty good about it.
As long as the Exhibitor themselves is diligent and has brought the forms required to ensure that, since you are an Oregonian you do not need to be charged, I am sure things will go smoothly for you... just double check though, and plan worst case scenario that, due to being a small operation, some of the vendors may not provide the service. I'll try and talk to our Exhib Manager or her Director about getting the word out on the issue to our Exhibitors, so they can prepare.
I think though, it is up to the vendor themselves how they handle it, and some places further north don't even recognize the concept!
As for Hotels, there is a tax, I believe it totals out to about 12.5% or maybe 10% of your bill. The wont remove any state taxes for sale or otherwise from the bill since it is a service not a good (and not a non-perishable good either). But I think the taxes are still less than Portland's tourism taxes.