I've had discussions in the past regarding the usage of rel=nofollow attribute of <a> tags. Some people argue that using this method allows you to do 'link sculpting' - that is defining which links should have greater weight to which parts of a website. The theory behind this is that with fewer links on a page, the ones that do not include the rel=nofollow attribute will pass on more linkjuice to the page that you want to promote. My counter argument is that if you reduce the linkjuice to a page, then the links on that page, including the ones that go to the page you want to promote, are less powerful, so the agregate effect is that you are damaging your internal linking powers by using rel=nofollow. There are a few exceptions to this, for example, if you have pages that have no SEO benefit - eg pages on a test url - then you can use rel=nofollow, but in general, I would advise you keep the full linking power of all internal links on your website. If the links go to an external website, then there might be a case to use rel=nofollow (eg in forums where anonymous posters can add links to unmoderated external content).

The video below addresses the question of rel=nofollow internal linking: