I use a plastic fry pan utensil to loosen the ice off the sides of my freezer. I think that would be safer than using a putty knife.

When growing up, when my Mom would clean the freezer, when every thing was out, she would set a bucket of hot water inside the freezer, then close the lid.  (Freezer turned off.) The hot water would help loosen the ice on the sidewalls.  Come back after a little while and pry the ice off.  Put it in containers, and go dump it.