If you're not going to pay for seats together, then make sure you get to check in early if you want to be seated together. Pax who haven't paid for a meal will be presat from the rear of the aircraft forwards. Pax with special requirements (eg wheelchair users) or who require specific seats (eg travelling with infants) will probably be presat, as will those who have paid for seats together and those who have paid extra for exit or bulkhead seats. On some flights this can mean up to 3/4 of pax will have been pre seated before check-in even opens, which doesn't leave a lot of choice for those who haven't paid for seats together.