In episode 114 of Semantic Mastery's weekly Hump Day Hangouts, one viewer asked if you need to nofollow an authority site to keep its link juice.
The exact question was:
An On-Page SEO Question: I'm building out an authority site, and I want to sculpt PR/Authority on my site via the internal linking structures and keep as much link juice as possible. However, I also want to link out to authority sites too. Would it be wise to no follow all of these links or not? What are your thoughts?