Joseph Rao, from my 2013 Manhattan College AP Summer Institute, writes in:
For AP Physics do we pay attention to the sign on Electric Potential energy? I know we do not assign a sign to Electric field and do assign one to potential difference when calculating potential some distance from a source charge or charged plate.
Hey, Joseph... the sign on electric potential energy is important, because objects are forced from high to low potential energy. A charge released from rest with -30 J of PE will speed up to a place where it has -80 J of PE.
Note this is different from electric potential (i.e. voltage). A positive charge is forced high to low voltage, while a negative charge is forces low to high voltage.