What you need is (a) a seat assignment and (b) to check in before the stipulated cut-off point for re-assigning your seat.

Here in the third world, where airlines do their passengers the courtesy of assigning them seats and telling them what those seat assignments are ahead of time (well, except for Southwest...), you shouldn't be IDB provided you check in on time.

If you book a ticket and they can't or won't give you a seat assignment, then take another flight ..... usually the only reason for that is the flight is oversold and there isn't a seat to give you....

In Australia, where QF doesn't let you select a seat and 'surprises' you at check-in with whatever seat the computer decided to give you, you're presumably ok if you're elite as I assume the computer assigns those seats first ..... but if you're not elite, I guess you've no way of knowing until you turn up at the airport that you don't have a seat...