Every modern gasoline engine, that I'm aware of, uses an ignition coil to generate "spark". Usually, a current is introduced into the primary of the ignition coil and then interrupted (by a solid-state switch of some sort) causing a spike in voltage at the secondary of the ignition coil. Depending on how the secondary winding is connected to the spark-plug wires (through the distributor), the resultant current could presumably be in either direction.
Is there a good reason for current to flow "towards" the spark plug versus "away" from the spark plug? Is it the same way on all vehicles? How about small engines (lawnmowers, snowblowers, chainsaws, outboards)?
All of this is significant because the cost of automotive timing lights is largely due to the cost of the inductive (clamp on) pickup. A positive or negative "edge" from the inductive pickup fires a Xenon flash tube which illuminates the timing marks. Some alternate way of developing an edge (relative to chassis ground) could be significant.