Not exactly. There are several different boarding priorities. Whoever checks in first within the same boarding priority gets priority. However someone with a higher priority can bump someone with a lower priority at the last minute regardless of time of check in. I think the different lpriorities go something like this

Employees on company business
Vacation passes
Employees
Employeee' kids
Employees' parents
Buddy passes
Other airline ZED passes

For example, if there are 2 empty seats and three people traveling on buddy passes listed, the first two that checked in will get cleared. If there are 5 seats available with 2 parents and 3 buddy pass holders listed, all of whom checked in 23 hours before the flight, then an employee walks up to the gate just before boarding begins, then one of the buddy pass holders (the one who checked in last) will not get a seat.