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Hx is a basic part of page structure. Bold is probably far less important but might still give cues about words on the page that matter more.
We use whatever we can to help the SE's determine what each page is about. Conversely however, if you go overboard on things, even simple things like on page elements, titles and META, I think you can send a signal to the SE's that a page is being "over SEO'd" -- and that IMO still has the potential to hurt rankings for such pages.