some people on the internet saying it is good idea to tell Google not to index:

- Wordpress posts Archive
- Tags
- Categories

because of:

a) duplicate content (posts text is visible both on category pages & archive pages versus posts itself)
b) too many links/low value content (tag pages)

but there are two options:

- tell google not to follow links
- tell google not to index pages

How to Nofollow or NoIndex in Wordpress

Nofollow: example by adding nofollow attribute to the category links in the widget. But how? its another question..

NoIndex: example by updating robots.txt file:
User-agent: *
Disallow: /tag/
Disallow: /category/

Nofollow/Noindex: A) in HTML header section of the page source code, one can add:

<meta name="robots" content="noindex,follow" />

it will allow following links on that page, but will not index that page

B) The plugin that is automatic nofollow and noindex on Wordpress is example: Yoast WP SEO

This tool can tell 403 (forbidden) error if you add an page within categories or tags you disallowed (noindex) to bots using robots.txt file (an example above)