“Does it make sense to use rel=”nofollow” for internal links? Like, for example, to link to your login page? Does it really make a difference?”
Matt Cutts answered that question in a video. He said that it doesn’t hurt to use the rel=nofllow attribute with links that point to useless pages. In general, it is better to avoid the rel=nofollow attribute on internal pages. Better use the noindex tag if you want to exclude pages from Google.
Unless you have a very good reason to exclude a site, it is better to allow Google access to all pages.