I had somewhat of the same problem... it wasn't that it was out of square...but that the plywood was "flexing"; I clamped the walls into square, added the trusses or spars or whatever you want to call them, sheeted the roof and released the clamps and all was fine....
FYI: You're allowed up to 3/16" out of square on EVERYTHING...

.... inserted to protect from the "i know its coming" comments about my frame...
Good luck on your build... looking great.