You also use the RF button to tell it whether to only trigger RF once (which might be a burst, if that's how you set it up) or allow it to trigger multiple times (which will eat AP).
'Not at all' might be a reasonable setting if you're sure the percentages will be off (tricky, narrow field of view for instance), you don't want to risk hitting some civvie who's near your probable lines of fire, or if you're low in the current magazine and want to wait for a better shot, or if you don't want to give away a location by shooting too early. If you're using the rocket launcher or you've used so many AP that you don't have a viable RF fire mode, you won't get the option, either.