That's good to know. I have found that there are usually still unassigned seats when I arrive at the airport. Seems like there is an unusually high number of travelers on Frontier that don't bother to check in online. YMMV and of course it makes sense to check in ahead of time. Worth also keeping in mind, however, that Frontier fills seats from back to front, so if you care about that you might monitor the available seats throughout the day and check in when you're happy with getting placed in one of the available seats left.