1. A method for caching web documents in two or more caches, the method comprising: for each query of a plurality of queries, where each query results in a plurality of web documents, each of the web documents of the resulting plurality of web documents having a corresponding tier score indicative of a position of the corresponding web document in the two or more caches, modifying tier scores of the resulting plurality of web documents by: determining a worst web document from the resulting plurality of web documents, the worst web document positioned in a cache of the two or more caches that has a highest access penalty relative to the access penalties for web documents of the resulting plurality of web documents other than the worst web document; modifying a tier score associated with the worst web document to indicate a higher priority cache, a higher priority cache having a lower access penalty relative to an access penalty for a lower priority cache; and modifying tier scores associated with web documents of the resulting plurality of web documents other than the worst web document to indicate a lower priority cache, a lower priority cache having a higher access penalty relative to an access penalty for a higher priority cache.