This statement may be true for some students or for some colleges, but cannot be universally applied. At my own college, the choice of beds/lofting vs unlofting, etc must be made by all students as a group as noted in the move-in procedures. The student who tries to pull a buttfoonery move by claiming a bed by being the first to arrive will not only incur the ire of the roommate(s) but also a call from the residential dean to explain the rules.
In theory, roommates will rotate beds at semester break, but may, as a group, decide to keep the status quo.