As I am moving to Oregon in a month I have the full printed version of the 2012-2013 Oregon Driver Manual. In Section 7 under Vehicle Equipment: Rearview Mirror; it says
"The driver must be able to see at least 200ft behind the vehicle at all times, and with all loads, on a straight, level road.....If a trailer or load obstructs the rear view, then outside rearview mirrors on each side of the MV are required. These mirrors need to be extended beyond the width of the trailer or load so the driver can see 200 ft behind the vehicle."
So unless you cannot see behind you with the rearview mirror, I dont see why you would need side mirrors. Hope this helps, and sorry for the lenghtly reply.