I used to use H1 for the main title on the title page, but the people on this forum scolded me for using headings for that purpose, so now I have a set of paragraph styles for the title page. In effect, that retired H1 in my books.
I use H2 for chapter titles, H3 for a significant (centered) sub-title, and H4 for a minor (left-aligned) breakhead. They are defined as percentages of the main text. You can see my style sheet on the blog at
http://notjohnkdp.blogspot.com/