If you are really confused as to whether you should nofollow some of your internal links, then you should not do that. Matt Cutts recently posted a video which says that links to internal pages like those of login pages should not be made nofollow. However, if your site provides any scope of link abuse like those in comments section or the user profile section, then those links should surely be marked as nofollow.
The crux is “If you are linking to your own site, make it dofollow, if you are providing opportunity for other users to link to their own sites or to other sites (those links that are not editorially approved) should be marked as nofollow”
Now, have a look at the video of Matt Cutts
Also See:-
Rel=nofollow Links for Widgets and Infographics
Syndication is a Valid Way to Get Links
Google Pushes for User Sharing of Sites
Where to link to the original source in the article?
Links from article marketing sites
Importance of Backlinks in Seo
How to Create Backlinks That Google Loves
How to Rank in Google
Google Search Quality Updates
Social Media Widgets for Websites
Nofollow Vs Dofollow
