Presenting the hostess with a gift is an age-old tradition. It is a form of expressing gratitude for arranging the event or party. Though it is not mandatory for every event, but presenting a gift is social etiquette and a nice way to appreciate her efforts. Choose an appropriate gift thoughtfully and not for mere formality.
There are no specific rules, so you should present something thoughtful to the hostess to let her know that her effort has not gone unnoticed. She will be extremely happy that you thought of her.