As long as all the policy servers are pointing to the same policy store or stores that replicate, you only need to reregister it once and either all the policy servers will use the new trusted host or if using multiple stores with replication, they will be copied over. You only need to run the smreghost on one Policy Server and make sure the new SmHost.conf file is written to the same location, as specified in your WebAgent.conf file, you should be fine. Also if your using multiple Policy Stores that replicate, it's best practice to write to one master Policy Store otherwise it could get confusing and multiple admins writing to different stores could overwrite each other's changes.