I think the general consensus is to block the upper grille first. For most cars, that seems to give more of an aerodynamic and warm-up bang for the buck than blocking the lower grille, although I'm not sure if that holds for the Prius.
Big +1 to vskid3's post: rather than overheating, it might be that blocking the grille could allow the system to shift into electric mode sooner. Just keep an eye on the coolant temp.
