Sub-categories automatically select parent categories AND allow parent-only option

I have categories and sub-categories in my taxonomy.
I want to allow users to categorize a node as either a just category (no sub-category) OR a sub-category -- and in the latter case I'd like that to automatically imply the category.

To allow the category-only option, I need to select the option "display parent terms as form items" . However, when people select the sub-category, they often don't select the category.

Can we make that automatic?

If there's a way to do this so it works reliably before the end of June, I'll paypal the person who makes it a $50 bounty. (I'm a little new to this, I hope it's OK to offer cash, and that amount isn't way off -- I don't really know what's appropriate.)

Thanks,
Patricia