I live in the Seattle area and looked at a cover as a basic requirement. Putting it on and taking it off is a bit of a chore but an ounce of prevention is worth a pound of cure.
During the summer I might decide to upgrade to covered storage but I couldn't justify an extra 160 a month, they also give hookups so you can keep it plugged in.
We will see. But yeah, cover is definitely worth it if you don't have a shelter over it.