How to use Headings to optimize your WordPress blog
You should use a few <h2> tags and as many <h3> tags as you need for the other headings in your posts. Doing this will allow the search engines to index the important information from your website better. A good document has headings and subheadings, because headings make it easier to determine the subject of the page.
For blog websites the best SEO practice is to set the title of your post in a heading1 tag. Most of the good WordPress templates do that. If not, you should modify the theme in order to achieve a correct output of your pages.