well, if you are wanting to support multiple pools you should use route-maps. route-maps will ensure that the router keeps a full translation of outside and inside addresses with protocol and port number. Where if route-maps are not used then only the inside translation information is maintained and there is not protocol or port number information maintained either. what could potentially happen in this case is that any external host could send packets to an internal host that got translated because there is not any protocol or port information associated with the translation.
Erik Rudnick, CCIE No. 9545
mailto:erik@kuriosity.com