If the available seat is a middle standard economy seat, I'd wait. You have a chance of getting assigned an MCE seat at checkin. Even if none are showing now, upgraders are likely to come from those seats so that might be all they have to assign when the time comes. The downside is that there is a very slight possibility that not having a seat assignment could increase your chances of IDB if the flight is oversold.
If the current available seat is an aisle or window, then you might want to take it if you have a strong preference to avoid middles.