Although a lot of people do it, there is no requirement that I know of for the official to signal the end of a period. The timer's horn causes the ball to be dead (unless a try for goal is released first). The official's whistle, then, is superfluous; and in fact, can lead participants to believe that a foul has occurred, which can just cause confusion.
If I've missed something in the manual, I'm sure others will let me know about it
