I personally like leaving the control in the hands of the user to start services, either in bootlocal.sh, the command line, or by the services app.
If an extension needs a particular service running, then it should use it's tce.installed script to start a needed service so the extension functions seamlessly for the user. For example, if Firefox needed dbus running, then firefox should contain "/usr/local/etc/init.d/dbus start" in its tce.installed script. I prefer that over having an extension that is a service start the service just because the extension is loaded.
That is my preference, though I see the logic in starting a service because it is installed for the sake of convenience.