Changing the user that nginx worker processes run under (Ubuntu 12.04)
As long as your new user (nginx in your case) has the proper rights, everything should work.
You have to change your user setting in nginx.conf
...user nginx;...and restart/reload your server. Link to docs.
If you use PHP5-FPM and get a 502 error after restart nginx, take a look on /var/run/php5-fpm.sock (Debian) :
root@ns353941:/var/run# chown nginx:www php5-fpm.sock
In my case :
nginx = new nginx user
www = group for web
nginx user belong to www, obviously.