There's still a benefit to indexing plugs in engines like ours but, like I already said, you're not going to see it (not unless you're pushing TONS of boost or your plug's gap is too large); it is there though. The airflow coming into the cylinders isn't nice and clean. The valves throw the air out sideways and while the air moving up toward the peak of the head loses a lot of its momentum from the collision there the air moving downward rolls around the top of the piston and tries to move up the other side, The piston's upward momentum presses all the air upward, but there is usually a pressure differential from one side of the cylinder to the other. Unless you've managed to pour the water into the dead center of the glass, it's going to roll up the other side.
I really hope I remembered enough of the specifics about that right... It's been a while since I've read up on that matter. If that doesn't make sense let me know and I'll see if one of my friends (who know a LOT more than me) can explain it more clearly.
13ft lbs...
Not too much... your method works well enough; Try it your way sometime and then hit them with a torque wrench and see how close you are.