Skulls as emblems... yeah, they occur on uniforms throughout history. There's some discussion to be had about whether the good guys would have skulls on their uniforms: https://www.youtube.com/watch?v=VImnpErdDzA
At least in Brazil , it doesn't bring a good feeling, because of "Scuderie Detetive Le Cocq"[1] , a secret killing organization formed by policeman, commonly referred as "Death Squads". They kill poor people for fun. Their emblem was a skull.
[1] http://pt.wikipedia.org/wiki/Scuderie_Detetive_Le_Cocq