The "conservation of pagerank" school has been debunked at this point. Linking out to useful sites helps your site.
Googles algorithms cause you to lose pagerank for using nofollow, or using robots.txt and linking to pages that are in it. On most sites, this pagerank loss far outweighs the loss from any external links.
Google algorithms also make it such that any loss of pagerank doesn't really matter. Lost pagerank seems to re-appear and be assigned to your important pages by little Google fairies.