Preparation and spraying cedar channel siding
If the channel of the siding has already been painted, and the paint seems to be adhering well, then you might be able to get away with just spraying over it after a good power wash. It has been my experience that paint in small channels binds better to the bottoms and sides than the surface, compared to a smooth flat surface. However, I'm a big proponent of excessive preparation. If you're going to take the time to paint, the last thing you want is peeling or flaking. It will add some extra work to your project, but I'd take a semi-stiff wirebrush and run through the channels quickly but thoroughly to put some extra teeth into the paint, particularly if the top coat is very smooth. Another option would be to construct a small sanding block, maybe with a 7/8" by 6" face and wrap a 120 grit or similar sized paper around it securing it on the other surfaces with staples. Then you can move the brush or block through the channels quickly scuffing enough to remove any signs of gloss.

Ultimately the decision may be a function of time, whether or not you have assistance, the type of paint on the exterior, the quality of the primer for the undercoat, etc.