Is indexing the spark plugs worthwhile? Or is it just another motoring myth?

It's not a myth, but for an everyday car it's probably not worth the hassle. You can buy indexing kits with various thickness washers.

Is indexing the spark plugs worthwhile? Or is it just another motoring myth?

Indexing is a complete waste of time with modern engines. American V8s and other engines with big compression lumps that mask the flame do benefit. In all engines with stock type plugs you can file the earth electrode back to the center of the main center electrode and taper off the corners to open up the flame path. Don't do this on platinum, silver or iridium plugs.

Cheers,
Alan