Actually, if you want to include the radio buttons, too, you could just do:
if ( fld.checked != null && fld.checked )
That's because ONLY checkboxes and radio buttons *have* a checked property. It will be null for all other types.
But of course nothing wrong with doing it as you showed.
Yes, if you wanted to add radio button values to the calculation, you'd need to put the onclick handler in them, as well.