here in minnesota, energy code requires you to apply "windwash
blocking". that is because the cold air strips the heat out of the
insulation where it is exposed to the outside. We used to use a scrap
of OSB between the truss heels, leaving about a 1 inch gap between it
and the roof. then, if you don't have enough room for insulation, you
will need a foam baffle fastened to the roof sheathing to allow an
airspace for ventilation. Recently, i have discovered cardboard
insulation chutes, which are stapled down to the top plate, then go up
to within an inch of the roof sheathing and fold over. provides a wind
wash block and a ventilation space in one.