Hi there,
As a general rule, you almost always want to "follow" internal links - this includes links between internal pages. To do otherwise could hurt your SEO.
Links do a lot of things. They create a path for search robots to crawl your site. They send off relevancy and popularity signals to ranking algorithms.
Is there any particular reason you thought of "nofollowing" your internal links?
About the only time you want to add nofollow to internal links is when doing so prevents search robots from going down the wrong path, such as so-called spider traps, or sometimes as a layer of protection to keep robots from crawling certain pages (not the best method to use)
Regardless, making sure your internal links are followed has many, many benefits. This is one of my favorite blog posts of all time - although a little dated, it applies to internal links as well as external: http://www.seomoz.org/blog/10-illustrations-on-search-engines-valuation-of-links
Hope this helps. Best of luck with your SEO!
