Do we have a citation ethics in science? Citations matter! Everybody knows this and Google Scholar (or Publish or Perish) shows it. Over the last years, I met people with different citation "ethics": Supervisors and professors require students to cite their own papers, even if they don't really relate or if there is a paper which would be more appropriate to cite. Journal reviewers nicely recommend accepting the paper for publication if only more two or three references (paper from the reviewer) are included. And even when an author cites a paper, there is often the question if the "idea" is cited or a specific "result" etc. E.g. there is a study showing that wrong citations are often inherited in papers that cite the misspelled citation, which indicates that the author didn't really read or access the paper and cites it only because of its general reputation. Similar to the IEEE Code of Ethics or ACM Code of Ethics, is there any Citation Code of Ethics?
Citations have only one practical purpose: indicating to your readers where they can find the published material on which you built the work that's described in your paper (methods you used, hypotheses that your work support or contradict, etc.). 

Everything else is just more noise in an already too noisy signal. 

Citations should not be used to show deference to senior researchers, to increase citation metrics of a person's supervisor, nor to "grease the palms" of reviewers or falsely infer relevance to a given journal. Please refrain from doing such things, they hurt science.