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I don't use citeulike myself, but I think it's important to make a difference between tags and keywords. Indeed, keywords are usually already included in the paper or at least in the abstract, so you don't need to retag them with duplicate information. Instead, it's better to use tags to give some personal context, such as why did you read this paper in the first place, or which general idea can you connect it to, or for which of your papers you used it, etc. In this case, there is no "wrong" or "right" tag, just some facts.