Continuing: The issue is to notice when deviation becomes distortion.

The other day I was recalling hearing Myra Hess play three Mozart piano concertos one afternoon in Chicago. I was about twelve. All I remember is that she was revered by my teacher, that she played with the music, and that her scales were unacceptable. Years later a violinist colleague recalled hearing Fritz Kreisler play in Chicago at about the same time in her life, remembering specifically how sloppy his playing was. We agreed that we would give our eye-teeth to hear such bad playing again.

Traditionally teachers go to great pains to make children conform to steady beats while many psychologists and musical thinkers (Jeanne Bamberger, among them) are coming to the thoroughly researched conclusion that there is no such thing.