I did that last winter for the first time and I think it helped (my winter MPG minimum was 52, vs 46 the previous year). I used pipe insulation on the lower grille until mid May (i.e. until the temp hit 70 during the day). Very easy to put on and remove. Black electrical tape (my Prius is black) worked fine for the top grille. I only took that off at the end of June. I'm going to put the top back on next week if the temps stay as they have been (i.e. mostly cool). The bottom grille block will come back in October. My dealer seemed to think the car looked great at my recent 60k servicing, so I don't think this regime is unreasonably dangerous to the engine - especially since the car is mainly used for commuting.