I'll start.
Booked a close-in AA flight LGA-ORD with BA for 2 pax. The only seats available were middle in back, so I decided to wait until checking in to see what would happen.
Results:
AA 331, 7 Oct, LGA-ORD: Checked in with agent and requested seats together, got seated in window and middle seats next to each other in row 8.
AA 336, 8 Oct, ORD-LGA: Checked in at kiosk, which auto-selected two economy plus seat in front and didn't offer another choice.
What I'll do next time: first go to the kiosk, and only if unhappy with seat selection will I go to an agent.