+1 I checked in online like 13 hour before the flight I my place in line A59 which I think is before the carriages.
i find it depends on route and several other random things i cant figure out.
I've checked in 23:00 before a flight (LGA-MDW) and was mid B's
another time i checked myself in (same route) and was placed at the end of A, while my wife who I checked in several minutes later was 20 spots ahead of me. go figure