Actually it's more like the original myth started with ABC type engines . Somehow everyone got onto this "you gotta run 'em HOT so all the bits will fit proper" which got someone to think that maybe lots of nitro would make them run hotter so the fits would wear in to suit. Then if you go to lower nitro it'll run cooler and the fits will change again and make it useless for the higher nitro. This doesn't happen but it makes a good story and eventually the story evolved into saying that as your ABC wears out you just go to lower and lower nitro levels so it runs cooler and cooler to keep the tight fits. But it's just a story.

In other words, to quote djlyon, "no truth. Urban myth"