Even though Feverfew, Tanacetum parthenium, can be quite aggressive in certain garden situations, it is not considered an invasive plant that escapes to the wild displacing existing native species. You are correct in finding out if this plant may be a thug, but the fact is that when we import a plant to our gardens, we don't really know if it will be invasive in OUR area. We are taking a real chance, and that's why it's gratifying to know that Americans like you are starting to ask the question: Might this plant be invasive?
Since you are already observing the carefree habit of this plant, continue to monitor its habits and take particular notice if it wanders away into a field or a neighbor's yard on it's own. Stay informed about the invasive plants that New Jersey targets for removal or observation. Study the Resources on the Web about Invasive Plants. Our collective vigilance can make all the difference, and as gardeners we should ask ourselves this question before setting any plant out in the garden: Does my wish to grow this plant outweigh the possible negative environmental consequences?