For consistency, our back judge has the whistle on any FG or PAT kick as long as the kick proceeds as normally expected. If it's blocked or is kicked toward the sideline, the covering official would handle the whistle as they would on any scrimmage play.
We blow the whistle only when the ball is dead by rule. At one time, many years ago, when individual referees operated their own fiefdoms, several used the "whistle at the kick" on PATs. That has been mothballed in our efforts to be more consistent and uniform.