It really depends on how you use them. I use my tag pages as navigation elements, when there are important similarities within posts of different categories. I let Google index those, because they're valuable pages (with the addition of some into paragraphs).
On the other hand, I have seen blogs where people tag posts with a dozen meaningless (to a search engine) things. In those cases, I'd no index the lot.
Ultimately, the answer will vary depending on how tags are incorporated into the overall structure of your site.