If you think you need a sub-heading, apply one of the lesser heading Styles; don’t just embolden and change the font size when you’re using Normal Style. That said, bold, underline and italics are fine for emphasis within a paragraph; they can be translated into the correct Braille code to emphasise the text.
This is a Heading
This is a Heading which just “looks” like one!
The above shows two almost identical-looking lines of text. However one has the Heading 4 Style applied, and the other is Normal Style text which has had the font size changed and Bold applied. They look the same, but only one will come into any alternativive media software as a Heading.