Even though in The International style of Dancing one can start on the first bar of music if they want to. It is preferable to have a four bar intro. An experienced dancer would start after the intro just in case the tempo might be a bit strange. There was a Waltz played for the final of the World Amateur last year which in the introduction was extremely strange. Anybody who has seen the tape will notice how the six couples didn't move till some eight bars had been played. And one in particular stood for even longer. Michael Barr and his co comentator Allison Lamb agreed it should be in a stage musical and not on the dance floor. Anyway four bar intro every time and from then on eight bar phrased is desirable.