"Bark side up" is the conventional wisdom for what I can only imagine is some academic reason versus how these deck installations look after a few years. The main reason proponents like this method is ironically to reduce cupping. OK, I'm not getting this. When it rains, and the board warps, water collects in the concave shape and pools there. Wood develops a memory over time and that's why you see deck boards in the permanently cupped position even when dry when installed bark side up.
I've read Forum posts where users think the permanently cupped shape is for "traction" when wet! (No, not in our Forum).
Another reason this method may be used is to avoid a defect called "shelling". Shelling can occur with the bark side down method and is a raised grain defect where late wood growth separates from early wood growth and creates a flat splinter-like defect after repeated wetting /drying cycles. This is more common in Douglas Fir and Southern Pine lumber. But all you have to do is be more select in your pieces of lumber selected to avoid this problem.
This method was also popularized when heavy green treated lumber was used for deck boards and that lumber was unevenly saturated causing warping problems. Anyway, I could go on trying to make excuses for "bark side up", but I just think it makes little sense to place your deck boards so they look like this after a few years.
