This one is often a little more difficult to implement. Themes usually organize Heading tags in their own way, and reorganizing them can be a hassle. Don't go crazy worrying about this, but if you can arrange it so that your post titles are in H1 or H2 tags, that would be ideal. If it would be too difficult to rearrange a theme's organization, and it would be easier to just put the title in an H3 for example, it's ok. Doing so won't destroy your SEO efforts, but H1 or H2 would carry more weight.