Most SEO plugins for wordpress disable indexing on wordpress category pages. So SEO of category pages in not important. This also helps to avoid duplication of the content in the post view and category view. However if you use excerpts instead of content for the post shown on the category view duplication of the content is not a problem.

To enable SEO for category pages, enable indexing of the category view. Add keyword rich summary in the description for each category. Plugins like all in one SEO will automatically use this description for meta description tag.

This method of doing SEO fpor category pages can be very helpful of the wordpress e-commerce sites.