Which way does current flow in (automotive) spark-plug wires?
It's not true that the spark is always positive to tip or that there is even much of a convention these days.
It's rare in modern vehicles to see a distributor, and the polarity of the HT voltage varies depending on the design and type of plug used. 
From memory older Kettering systems were typically designed for the plug tip to be positive on the basis that this reduced spark erosion of the tip (the tip was hotter) and the spark fanout was toward the ground. Today you see both positive and negative tip sparks, especially when you have one coil per two cylinders. Look here for a comment from a sparkplug manufacturer.

One of the funny things is that magneto ignitions in aircraft engines normally always hit one plug positive and the next negative because of the magneto magnetic structure. It's the same in small two cylinder tools as well.