No, the police should not be.  This not the proper role of the police.  The police do not have the correct perspective on the issue and do not know as much as the prosecutors do.  One of the major reasons to plea bargain is because you do not think that you would be certain to win the case at trial.  It is the prosecutors who know which kinds of cases are most likely to win in court.  The police do not have the legal experience to make this determination.  The role of the police is to arrest suspects and collect evidence, not to determine who should be charged and how.