I would recommend you do just to help protect yourself a little. You don't have to have a medical authorization to use cannabis in Washington. All you need is a medical history that shows you have the diseases that are listed as being covered in Washington. You will have a higher chance of getting drug into court if you don't have the authorization but as long as you have proper medical documentation you don't need the authorization.
Usually the easy way to show you qualify is that using opiate based pain products makes you vomit. Its called opiate intolerance and its very common. But you will need documentation from a doctor that you've tried a couple of pain killers and they don't work.