Switching seats with members of your own party

Hello! I am going to be choosing standard seats at the 48 hr mark (card member) for my party of 6 tomorrow. I understand we won't get seats all together, but I am hoping to secure 3 groups of 2. I assumed I wouldn't actually have to pay attention to which seat I chose for each person, but could quickly click through the names and choose 3 groups of 2 seats. I thought we would secure the seats and then sit wherever makes the most sense (one parent with each younger child, and teenagers by themselves) as long as we are only sitting in seats assigned to SOMEONE in our party. AI says it actually matters, though, because of weight, balance etc and we aren't allowed to switch among our party. Is that true?