The biggest reasons for using nofollow are about duplicate content on your own site. Multiple links that have to be followed and indexed that go the same place can get you site banned from some of the smaller search engines and search services. On your homepage you want no follow on all multiple and external links, but they should be followable and indexable on your single page. If you use wordpress the best plugin combination for this is nofollowthosedupes and nofollow free