The other thing to note is that anything you input in Source isn't permanent. So it's usually one of the last things I do. I'll write my page, make whatever edits I need, then go to Source and enter whatever special codes I want. Then I save the page and take a look at it in the story viewer.
If I make no further changes, the centering will stay there. If I noticed I misspelled something and go back into that page, any special formatting keyed as code in Source will disappear. I will have to reenter when I'm done.