FWIW, a few weeks ago I bought an F ticket by phone about 50 minutes before a flight, while in a cab to the airport. I was too rushed to ask for an assigned seat. I called my wife to ask her to check me in onlne (I was worried about the 30 minute check-in limit which may or may not exist). She got a message about not being able to check in without an assigned seat, and she couldn't pick a seat. I ended up in the seat she tried to pick for me, perhaps because it was the only seat left.