I think it depends on what you mean by "nice" and what you mean by "belief system". My son believes in Santa and that he'd better be nice or he won't get any presents. So yes, it makes him be a little nicer to his sister.
My belief system is based on "Come from a place of love not fear." I try not to judge others but "Be in acceptance of all that comes my way". I am a nicer person now, than I was when I had the witnesses belief system.