The problem is called "Truss Uplift" and is mainly a concern in colder climates. What happens is that during the winter, the bottom chord remains relatively warm as it is burried in insulation (e.g.: the trusses in our house have bottom chords mainly made of 2x5 stock - yes two by five - while the insulation is at least 14" deep).
The other members of the trusses are up in the cold and they tend to contract in the cold. This contraction has an upward force on the bottom chord. This can lift the bottom chord, and your ceiling, away from partition walls.
Our partitions are framed with steel and were done AFTER the roof was in place. For steel, the top chanel is attached to the ceiling structure, which was a concern for the second floor as the trusses span the entire 33' width of the house. Our method was to use two channels: one attached to the bottom of the chord, and the other attached to the studs. The stud channel was inserted into the truss-attached channel. This provides lateral support while allowing truss lift to not damage the steel stud walls.
This does not prevent any gapping of the drywall between the ceiling and the walls, but after three winters we have no visible cracking in those joints. I suppose our trusses have been designed to minimize uplift.