Dan:
If you buys the chains from Les Schwab (PNW tire outfit) and don't use them, you'll get a refund. The law here doesn't say you have to carry chains, if you have awd/4x4 and tires with the snowflake symbol, you're covered. If they go to condition C, which is chains required on all vehicles, then your rental company will have to just deal with it, because you'll get a ticket if you don't chain up. By the looks of the weather, you might well need chains. I suggest cable chains from Bi-Mart (local dept. store). Should run $20/pair. Make sure they are fairly tight to avoid damage to the car. Use
http://www.tripcheck.com to keep updated on road conditions in Oregon.
And above all, have a safe trip!
Cheers,
Paul.