Pro/cons of checking/unchecking parent category when using child categories?

When you use child categories in WordPress, parent categories aren’t automatically checked when editing the post.

You can check the parent category if you want, but I couldn’t find much info on the differences between checking parent categories VS. keeping them unchecked, so I did some tests.

Here are the results:
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1) Permalinks change.
With parent cat CHECKED: /parent-cat/post-name.
With parent cat UNCHECKED: /parent-cat/child-cat/post-name.
(I use the following permalink structure: /%category%/%postname%/)

2) Breadcrumbs change, albeit differently depending on theme.

2a) Breadcrumbs with Schema theme:
With parent cat CHECKED: Home > Parent Cat > Child Cat > Post Name.
With parent cat UNCHECKED: Home > Child Cat > Post Name.

2b) Breadcrumbs with Atmosphere Pro theme by Genesis:
With parent cat CHECKED: Home > Parent Cat > Post Name.
With parent cat UNCHECKED: Home > Parent Cat > Child Cat > Post Name.

3) Posts are archived under both child category AND parent category, REGARDLESS of whether parent category is checked or unchecked.
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So… I’m still confused as to which route to go. What other differences or pros/cons are there for using or not using parent categories? Are you using it or not, and why?