Nofollow on external links

I've used no follow for links to pages such as aboutus, contactus, feedback, sitemap.

Is this correctly used.

Can nofollow be used for external links to not benefit the sites im linking to ?

It depends of your structure and the important section of your site that you want to "share" more PageRank than other (contact, about,...)
nofollow should be used for external link for this propos and anti-spam

Your site will benefit on not to pass the pr link juice to other site. It's a good practice to put nofollow attribute to outbound link.

Really, nofollow is just used to prevent you from passing on Pagerank. You still pass on actual link value based on the anchor text. I've seen this myself by ranking specific blog posts of mine for specific keyword phrases soloy by using nofollow blog commenting. You can try it out yourself on a not so competitive keyword.