I would like to see the stiffener shown on the flat oval duct. Most companies ship the oval duct without the stiffeners attached to prevent shipping damage. When it gets to the field it would be very nice to have the ability to show where the stiffeners are to be installed. It would also serve as a reminder that stiffening is required so it does not get missed by the field installers, who many times are just going by the prints and are not aware of the project specifications.
It is usually just an angle top and bottom screwed to the flat span so it does not seem like it would be a big deal to show on the drawing.