Look at figure 2, floating corner at the ceiling: http://bestdrywall.com/files/ReduceCallbacks.pdf
Instead of nailing with-in 7" of the wall, install a 2"x2" metal flashing on the wall only, then screw the ceiling drywall to that. No screws into the corner ceiling backing or framing. It moves with the wall, especially if the next rafter is 2' away. More room to flex more.
Be safe, Gary