I bought a Moroso indexing nut. You just screw in an old plug that you know is indexed for a certain cylinder, mark the position on the nut, then find a new plug that screws into the same position. The nut is good and wide so you can mark a scratch on it and a cylinder number. A hell of a lot easier than indexing on the cylinder itself.
Another reason for indexing, other than piston to plug clearance, is to ensure the spark is facing the gases and the strap is not going to interfere with any ignition of the gases or flame travel.