I've noticed the same thing as ciaohound and the same thing on all the ULCCs including G4 and F9. I've checked in a T-24 and all the way up until arrival at the airport and haven't found any particular logic although I have found that when checking in earlier we've been more likely to automatically get seats assigned together. If I were you I would check in at T-24 and if, by chance, you aren't assigned seats together, you can still pay to move together or arrive at the airport as early as possible, before people start checking in at the airport, to request seats together. I've never paid for seat selections and never not been able to sit by my traveling partners.