Thanks. I did just read 334.15 online at
There seems to be a gray area in there somewhere in terms of the "protection from physical damage" issue. It doesn't seem to be clear. Apparently, it is specifically okay to staple NM cable along running boards that go across the ceiling joists in an unfinished basement.
But, there is nothing specific about the stapling of NM cable to a running board that goes vertically down the wall to an outlet or switch. I guess that means it is allowed unless there is some reason to believe that the NM cable there needs to be protected from physical damage.