Setting an events non-interaction to "true" simply means that triggering that event will have NO impact on bounce rate.
I have seen cases where someone sets all the events to "false" effectively breaking the bounce rate of an entire site. When in doubt it's always safer to set non-interaction to "true", unless you want that interaction to have an impact on bounce rate. just be sure that event interaction isn't going to break the bounce rate metric for users who really do bounce.
Your example for adding a item to the cart without visiting any other page and then leaving is a prime candidate for setting non-interaction to "false". In that example a user leaving would not be tracked as a bounce since they did add something to the cart and the add to cart event has non-interaction set to "false".
