Dean, there are several schools of thought on this problem; some believe you should not tie them down, other's do. I agree with the latter, primarily for structural integrity reasons, but also because this ongoing motion will mean popping drywall loose over time. The trusses are often toe-nailed down originally, but these work loose rather easily.
You can use metal "L" brackets or mending plate in the attic to tie the bottom members of these trusses to the top plates of the walls. The use of long screws on the top plates will ensure you get a good hold on what's usually a doubled top plate.
If you chose not to tie them down, your best bet it to put crown molding at the ceiling line, attached to the ceiling joists, not the wall. This allows the crown to "float" with the trusses, sliding up & down the wall, but always covering the gap. This is more successful when using bed molding, a slightly smaller version of crown, or a cove molding. The larger crown is hard to keep tight to the wall without nailing it.