No, registration does not make anything about your app publicly viewable. Upon registration, the software gives you the keys and creates a dashboard page for your app with URL
http://stackapps.com/apps/oauth/view/xxxx. This page is not accessible to the public.
Also, the sidebar of registration page explains:
Because it's the neighborly thing to do. We like to know who is using our API, and how, so we can have the metrics we need to support your application and improve the API together.
Once it's ready for public consumption, we'll help you promote your registered application here on Stack Apps.
Registering achieves the primary goal, stated in the first paragraph. If it turns out that your app will never be ready for public consumption... that would not be unusual. A lot of products are developed that never ship.
Disclaimer: I do not work for SE. The post summarizes my own observations of StackApps.