Careful documentation can be time-consuming and dull work, but there's no getting around the fact that this work is essential to good scholarship. Endnotes and other forms of citation serve the purpose of drawing a clear a line between your work and that of other scholars. As any of you know who have found interesting ideas or sources in the notes or bibliography of an article, they can be valuable in pointing out new directions for readers to take in their own research. Obviously, they are also important because they allow readers, including your professors, to verify your work. You need to document your sources scrupulously, not only for the sake of intellectual honesty, but because the consequences for failing to do so can be significant.
Many professors mark down severely for improper or sloppy citation.
And incomplete documentation is a violation of the honor code. If you don't cite all your sources, you open yourself up to accusations of plagiarism, meaning that you appear to be passing off somebody else's work as your own. Also take a look at the Princeton University publication Rights, Rules, and Responsibilities. There are a few basic rules of thumb you can use to judge whether you need to cite a source. If you quote an author, even if you are only borrowing a single key word, you need to tell your reader the origin of the quotation.