I was surprised to find that this is not an option already.
We could really use a Project system configuration setting that allows us to default all new projects "Completion mode" to automatic.
Because not all projects are created from templates we have to manually report on and track down projects where all tasks are marked 100% complete, but the project is not closed/completed and manually change it. Yes, sometimes there are open approvals and/or issues/requests on the project. But in our instance, more often there are not.