I use to index them. Now I don't. Haven't felt or seen any difference in hp, torque, response, ect. There are so many other things that are more important. I just don't waste my time with it any more. I worry about proper gearing and carb settings.
Although I have found that when the engine becomes hard to start, a new plug makes a world of difference! That is why I don't index them, I'm changing plugs often and checking them often. Don't want to waste time with different shims and washers to get it "just right." I'd rather spend my time on the carb, or checking all those bolts that can rattle loose!
Paul