If you want to sit together it's always nice to have the reassurance of booking in advance, particularly if it's a long haul flight. I've seen people check in late due late transfer from holiday resort and then made a scene at check-in when not allocated adjacent seats then repeated the scene on the aircraft , because they hadn't booked it. If you don't book it you are not guaranteed it. You could always take a chance and hope the flight is not full and find some spare adjacent seats. On some Thomson long haul flights you can check in the day before from UK which might help on the outbound trip.