1. A computer-implemented method of assigning a document identification tag to a new document, the new document to be added to a collection of documents, the method comprising: on a computer system having one or more processors and memory storing one or more programs for execution by the one or more processors: subdividing a predetermined set of monotonically ordered document identification tags into a plurality of tiers, wherein each tier is associated with a respective subset of the set of document identification tags, and wherein the plurality of tiers are monotonically ordered with respect to a query-independent document importance metric; receiving query-independent information about the new document, the information including a value of the query-independent document importance metric for the new document; selecting, based at least on the query-independent information, one of the tiers; and assigning to the new document a document identification tag from the respective subset of document identification tags associated with the selected tier, the assigned document identification tag not previously assigned to any of the documents in the collection of documents.