I will be frowned upon, but I always pay for seat selection in advance for us as a family of 4. Kids are 14 and 12, very well travelled and will never cause a fuss - however we just prefer too have them sat with us. I can't afford J cash prices, so everytime we go J its with 2-4-1 and Avois, or just Avios.
Seat prices in CW are crazy, but I blindly add that cost onto the overall tax etc, and for me its still a good price for J travel. I know i'm the kind of customer which means BA always have a market for selling seat reservations. I just find the stress levels probably drop a bit overall just having certainty and to avoid, possibly asking if others would consider move seats.
Costs are high, but overall with a 2-4-1 paying for seat reservations its still a good deal for me