Since the Service Manager Tool relies upon the StartupItem, it is necessary to install the StartupItem, regardless of of you want to have the system start up PostgreSQL on every boot. Fortunately, OS X provides a mechanism for dealing with this very situation.
In the file 'hostconfig' found in the root 'etc' folder ( cd /etc/;sudo vi hostconfig ), you need to add a line to tell the StartupItem to not automatically start. You can do this by placing the line 'POSTGRES=-NO-' at the end of the hostconfig file. Once this file has been saved, the next restart, PostgreSQL will not start automatically.
In order to manually override this, there is a new parameter accepted by the startupitem script that you can use to force the issue. From the command line, you can issue the '/Library/StartupItems/PostgreSQL/PostgreSQL start MANUAL' command to force the startup. The Service Manager will issue this same command.