In symfony 1.x, you can enable a plugin in the project configuration and in the each application.
A good rule of thumb could be to put common plugins in the project configuration, as they are shared accross all applications, and specific ones in the concerned applications… but it’s not!
In fact, some problems may occur with the symfony tasks as they use sometimes the first application they find, and sometimes no application at all, so most of the time the applications plugins are not available for command line tasks.
This is particularly annoying for
plugin:publish-assets which must publish the assets of all plugins, or
propel:build-model which needs the
schema.yml files of all plugins too.
So the true real good rule of thumb is to enable all the plugins needed by any task in the project configuration, and then to sort the remaining plugins following the first rule above.