That helps your readers search each page, and makes them happy - and making your readers happy increases SEO.
With a Blogger blog, what makes your readers happy increases SEO, also. With a blog, you make your readers happy by providing content, for them to read. And you make them happier, by organising the content.
Your blog is organised by posts - and your posts can be dynamically grouped.
You don't need large and small headers, to indicate relative importance of a section of page. With a blog, you have multiple groupings, and content titles.
A website has only static pages. A blog has both dynamic pages (aka "posts"), and static pages (aka "pages") - and both may be indexed, and contribute to SEO.
A static website page can be viewed as a dynamic blog page, in different ways.
With a Blogger blog, each post can be the equivalent of a single section of a static website page. You can view the posts dynamically, in dozens of combinations.
- Most recent posts. The home or main page.
- Older posts. Archive retrievals.
- Related posts, in many groups. Keyword / label searches.
- Single post. Each individual post page.
You can eliminate most recent posts, using a static main page - or de emphasise most recent posts, using jump breaks. If your posts are not relaxant to the date, you can change, or hide, the dates of the posts.
Your readers can search your blog, using keyword and label searches. Give them the choice.
Your readers don't need large and small headers, they enjoy content. You should do, with your blog, whatever makes the blog look good. And write content, which makes your readers happy.
"SEO" comes from your readers. Google will reflect "SEO" - not determine it.
With static content, as you get in WordPress, you need relative header sizes, to indicate what sections of a display page are more important. You can only do so much, with a static web page.
With dynamic content, as you have in Blogger, each blog owner and reader can make their own decisions. Each blog owner can style their content, dynamically - and each reader can read the content, as they wish.
You can emphasise sections of pages, dynamically.
If you decide that the posts are more important, in a label search, you can add code to the template, to make gadgets - or the entire sidebar - disappear.
That's a way of making the labels index less important. You could use that same principle to make various gadgets less important, on various pages.
With dynamic content, you don't need to limit yourself to changing Header Sizes. You can make whole sections of content appear or disappear, depending upon what page the reader is viewing.
Be thoughtful. Make each page easy on the eyes.
If you're looking for a quick fix to your blog, you could use "H2" for post titles - and "H3" for gadget titles, and for titles of sections, within posts.
Since multiple "H1" on one page will confuse your readers, there is a reason to not use "H1" for post titles. Multiple "H1", in an index (archive / keyword / label / main) page, will not create a pleasant experience.
Google indexes content in posts, and optionally, in pages. The content of the gadgets is only interesting to human eyes. "H1", "H2", "H3" is all the same, when separating gadgets and posts, to Google.
Remember why you publish a Blogger blog.
Keep it all in perspective. Write for your readers. Let Google index the blog, based on what your readers enjoy.