Do electrical junction boxes in the attic that are mounted above the insulation and on the roof truss require a cover plate?
Yes, every box requires a cover plate.  You can't have a cavity with individual wires and wirenuts enjoying a view of the attic. 

And a steel box needs a steel cover plate. 

Blank cover plates are readily available for less than a buck. There are only a few styles and sizes and I just keep one or two on hand.