The seating software at check-in automatically fills aisle and window seats first. Then fills middle seats from front to back (ex: starting at row 12)

On a pretty full flight if you want to have an empty middle you are best to choose a row near the very back of the plane.

They can't block a seat for you unlike some other airlines.

In an ideal world middle seats next to SE and E's should automatically be the last seat to be filled.