For most flights, you should have a pre-assigned seat days prior to departure, unless the flight is really full/oversold. My experience is most of the time, I am assigned a window or an aisle right out of the gate (most data point on transcon flights).

Assigned seats on BE tix can be changed at check-in. I have been assigned plenty of middles, but have never sat in one voluntarily (close to 100 flights). Even on a full flight, plenty of seating changes will take place after check-in opens (e.g. Silver moves to E+, other pax upgrade to preferred seating or higher), periodically check seat chart, and you are bound to score a seat other than middle.

if a regional jet, then there would be no downside on a BE ticket as far as seating preferences are concerned (2-2). Also personally, for short/mid haul flights, I really don't care one way or the other where I sit.