If we do it as a `before_save` then there are some cases where we don't get the behavior picked up as we would like.

Example:
I have a published PageA and published PageB.  If I make PageB a child of PageA and click "Save changes" then PageB isn't a child in the `before_save` callback but it is in the `after_save` (I think there was also an issue w/ the publish scoping in `before_save`.

I can't remember which specifically was the issue but it seemed like the objects are in a more reliable state in the `after_save` callback to determine the  correct behavior.