Frontier definitely assigns seats from the back at the 24-hour mark. Makes sense, since they're still trying to SELL paid seat assignments during the check-in process. So there's zero incentive to check in at the 24-hour mark. It's always better to wait a few hours.

What I don't for sure is what happens when all the "cheap seats" behind the exit row get taken. If you have a party of 2 or more, will they give you the more expensive seats or break you up into middles and such? I haven't been willing to wait long enough to find out -- I don't really care if I'm in row 22 or row 12 -- I just want to avoid the random middles. This blogger did a more complete experiment:

https://www.pointswithacrew.com/get-...frontier-free/