John,
I am assuming you are talking about installing cedar shingles, correct?
I do not think it matters if they are going to be installed on the roof or the wall. There will always be moisture between the the surface they are going to be attached to and the back side of the shingles. Be it from rain or interior moisture from the house. Therefore I always install two layers of strapping. In your case I would go vertical with 16 O.C. and then horizontal with using the exposure of the shingle as the O.C. distance. Probably 6" or so. Since the strapping creates a gap at the bottom ideal for rodents or insects I usually install Core-a vent which closes up the opening but still allows for air to infiltrate and water if any to run of. I would do the same for clapboard or board and batten siding!!
There are other products like "cedar breather" and some housewraps with a rainscreen attached that apparently do not require strapping because of the webbing. I am not so convinced of them though because they do not allow for enough air to infiltrate and dry the wall covering, since they are only about 3/8" thick and once you nail the siding they get squeezed flat even more. On top of that the webbing blocks/ hinders the air flow. They are more quickly installed though.
