There's no way to force it to always be on, but you can use the "Take Snapshot" item under the Perspectives menu to add it to the various views in the app.
So, you'd switch to the default view under that menu, change the setting, then save a snapshot to preserve the new setting. Repeat with Inbox, Project, and Context views and from then on, those views will show completed items.
You can use this same procedure with other View menu settings, and you can make new custom views - called Perspectives - in this menu, as well.