Are you talking about a WordPress site? For a bog-standard blog, your tag, category, and archive pages will all likely carry content that is found elsewhere on the blog so you are probably not going to gain much from having them indexed. If you are doing something a little fancier, for example using an e-commerce plug-in etc., then you might not want to noindex categories as you you might have unique content on those pages.
I agree with maddenitrous. A good concept with a WordPress blog site is to use categories for site navigation and organization, then use the tags for keywords. With that setup you would want to index the categories and not index the tags. Also make sure that you don't duplicate categories in your tags or tags in your categories.
It's strongly recommended for a Wordpress sites to do this! I am just not sure how many tags is the maximum Never thought about this