You should not use the Headings to change your font size simply to emphasize text. Heading 1 down through Heading 6 are used to change the font size (and perhaps the font colors, or even the fonts themselves), but their usage should be reserved for actual headings.
Search engines use your headings to index the structure and content of your web pages. It is important to use headings to show the structure of the content on your page or post. Heading 1 is for the most important heading, Heading 6 is for the least important heading.
Users skim your pages by its headings too. Using headings helps your users move intuitively through your content. They won’t be able to do this if you make entire blocks of content display as headings.
When you need to emphasize text, use bold or italics. Never use underline for emphasizing text on the web–your users will mistake it for a link.