For people who do not abuse the preboard option I do recommend checking in normally as you would 24 hours prior to departure, then if you do not like the assigned boarding position, purchase the upgraded boarding. This is a better way to guarantee a good boarding position when compared to EarlyBird which is not primarily intended to improve boarding positions. This method also allows you to evaluate your free option and decide if you want to purchase an upgrade. The cost difference between EarlyBird and upgraded boarding is minimal on non-stop flights.
--TheMiddleSeat
... View more