Originally Posted By: Virtus_Probi
On Southwest, you want to check in as close to 24 hours before your flight as possible to be able to board as soon as possible...unless you pay the extra money to have SW check you in and get ahead of everybody but the business and high mileage travelers. You don't actually get an assigned seat, of course, but you can make sure you can sit with family and/or friends (if applicable) and also be sure to get some overhead bin space.
On other airlines, I've always been able to pick my seat at the time of purchase, so check in didn't matter...unless I had to switch flights for some reason.
Until very recently, I was a road warrior who flew 40 weeks a year, and this information is correct.
Southwest is the only one where you gain something by checking in literally the SECOND after you are 24 hours prior to your flight time. The Early Bird add on.. which is I think now around $15/20 per way helps to keep you from getting stuck in boarding groups C and D, but a lot of it just depends on how many people ahead of you are A-List Preferred or A-List. Certain "business" flights, like the 6AM Monday outbound from Dallas to Houston is going to be chock full of frequent travelers so you could still end up boarding later.
I usually was A-List preferred, and I still set alarms to remind myself to check in 24 hours prior to a flight.. and I was very often in positions A16-A20. The first 15 positions in group A are reserved for Business Select.