In NC you'd probably be fine without battens (most of the time at least) as long as the boards fit together well without too much of a gap. Personally I'd put battens on at *least* the side the worst weather comes from in Winter, though, probably all four sides.
I wouldn't recommend plywood for the battens, though; REAL wood will last quite a lot longer in this application. You should be able to get 1x2 or 1x3 furring strips for cheap - they will be a bit curvy/twisty/weird but the majority will do just fine as battens. Recommend predrilling your nail holes to avoid splitting.
Good luck, have fun,
Pat