Should I only optimize tags on my Web site's home page, or on other pages, too?
Definition and proper usage of Web page tags is a critical cog in the search engine optimization wheel. However, determining the most effective tags is by no means an exact science; but in general, the tags should reflect the site's main topic or message and also for the page itself, while at the same time mirror words or phrases that actual search engine might use. Tags should be used throughout the site: in page body, Title tag, Meta tags, headings, alt attributes, and link anchor.
When defining your tags, you must attempt to put yourself in the search engine user's position and try to imagine which words and phrases he/she is likely to use when searching for your particular type of website. A well-defined and wisely-used set of tags will encompass a fairly limited number of words or phrases actual Internet users might use to search for the site. In general, optimizing a Web page for more than two or three keywords is not feasible.
See Choosing the Best Keywords for your Website for more information.