You are probably better to checkin at the airport.
Allegiantair does not allow you to pick a seat for free 24 hours before flight like other airlines. From what we have seen, if you try to check-in on line at the 24 hour mark, you will invariably be assigned really cr@ppy seat at the back of the plane. This seems to be an attempt to get you to purchase seat assignment in better seats. However if you don't purchase a seat at that time you end up getting the cr@ppy seats. Our last trip, my buddy and I booked separately, he went to online check-in at the 24 hour mark and he was assigned seat 33E (middle seat right at the back of the plane) and he didn't want to pay for a seat. I didn't bother and just checked in at the airport, I was seated in 14D (aisle just forward of the wing). Sadly there was no one else assigned in my row, so once we were airborne my buddy came and sat up in row 14 with me.
We usually only purchase the seat assignment if we have a really early flight (like 6am), worth the extra $12/seat to get everyone seated together early in the morning and we don't feel the pressure to be at the airport the recommended 2 hours before flight time. (Spend the extra $12 and sleep in for an extra 30-60 minutes - worth it for me anyway)