Personally, I don't think it is so much the rule (if any) as it is the preference which is at play here as far as your question is concerned. Boldface Italic Quotation marks Underlined All of the above are devices that we use to suggest attentions, foucs on something special, or contrastively, to isolate a specific context. As long as we are consistent in our usage, I think it's ok to use any of the above.
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