We are not aware of any Washington state law requiring the addition of all owned cars to a policy. If your agent claims there is -- and we hear this from consumers with some frequency -- he or she should be able to tell you what section of law it is.
There is a section of state law (RCW 46.30, regarding mandatory liability insurance) that requires drivers operating certain vehicles on state highways to be able to show proof of liability insurance if asked to do so by law enforcement officers or a court. However, simply owning a vehicle that's not driven doesn't appear to require proof of insurance under that law.
If you're driving it around, yes, of course get insurance on it. The fines for driving without insurance are steep, and the potential losses in an accident could be much steeper.
For more specifics on the mandatory insurance law, including requirements and types of vehicles that are exempt, please see our mandatory auto insurance page.