Headings, Titles, and Labels
http://www.usability.gov/pdfs/chapter9.pdf

Usability.gov
2006

Abstract:

Most users spend a considerable amount of time scanning rather than reading information on Web sites. Well-designed headings help to facilitate both scanning and reading written material. Designers should strive to use unique and descriptive headings, and to use as many headings as necessary to enable users to find what they are looking for--it is usually better to use more rather than fewer headings. Headings should be used in their appropriate HTML order, and it is generally a good idea not to skip heading levels.