Originally Posted by MikeBOS

It doesn't matter as long as you're there before they process the upgrades. I'd make sure to get on the list at least 1/2 hour before the flight...if you checked in 24 hours ahead online, you'll end up near the top of your status group. I've been told that fare class also has something to do with the ranking on the airport list (not the advance list), so that might also affect your place in line. That said, I've never failed to be at the top of the list in my status group when checking in online 24 hours ahead.