I'm not a expert, but as far as I understand the system of nginx and php-fpm both maintain a pool of worker threads.
If you have multiple websites that use their own priviliges both services will start at minimum 1 process for each site, to serve incomming requests.
Starting new processes is very expensive in regards to processing time, therefore at least one worker process for each site should stay in the memory of the server, waiting for new requests.
If you kill those processes, you will only raise the response time of your websites.
Last edited by harkman; 2nd July 2013 at 15:01.