I understand others have different feelings, but I can't imagine buying seats in the last few rows of the plane. I have flown F9 a number of times with family members and have always gotten seats together. I never buy seats, I just go with the "I'll take whatever" option. 24 hours before the flight, if you check in, my experience is that they assign seats starting aft of the exit rows, going from A to F. If you watch, you see the seats fill as people check in. You can then check in and get the next X seats, with X being the number of people in your party. Even though I would never do it, I understand buying seats toward the front of the plane, but not the back. The fear of not being assigned seats together on F9 is very overblown in my experience.