I agree with setting the initial values of the spans, but am not jazzed about reusing the event handlers in this fashion. They're should have one function: be an event target.

Instead, I'd prefer:

function updateValues() {
  // ... set all the reporting spans
}

function onFrequencyChange(evt) {
  // ...report the frequencychange event
  updateValues();
}

// initialization code
updateValues();

See the Battery Status js, which is structured in this way.