If I remember correctly Shimano recommends lacing with trailing spoke heads in (flat part out) for rear drive-side whereas Sheldon Brown recommends the opposite, so take the reasoning that sounds best to you and run with it. Ultimately it probably doesn't matter much, especially if your chain doesn't fall off to the inside which it shouldn't if everything is in proper adjustment.
I just looked at two sets of wheels that I have built which are nearly identical. On the older set I have drive-side trailing spokes with heads facing out, on the other set they are facing in. I have no recollection of why I made the change. I do not notice a difference and I haven't had problems out of either set. If I were going to build another set right now I would probably do trailing heads facing out, but as Sheldon Brown pointed out:
Note: This is not an important issue! There is a sizable minority of good wheelbuilders who prefer to go the other way around, and good wheels can be built either way.