I flew Swiss from ATH to ZRH a few months ago. Didn't bother to do online check-in. On check-in at the desk, the missus and me were assigned seats 8 rows apart. When I requested the desk agent to seat us together, she said she couldn't, the flight was full. On board, I discovered that I had been assigned a middle seat in between an elderly couple.
Lesson learnt: always do online check-in.
Not that it helped on the return. Even though I did online checkin as soon as it opened, on boarding found that they'd switched aircrafts and reassigned us new seats at the gate. However, we did end up with bulkhead seats, either by chance or because the software rewarded us for checking-in online.