All things in moderation. If you add an external link where one makes sense and don't link to that domain sitewide, and if the site is of decent quality, you have nothing to worry about. Nofollow isn't needed on those types of links.
A certain amount of linking out is expected and hording is frowned upon since it would make no sense to rank one page above another based on how much PR the site hordes. I'd be very surprised if linking out to high quality content didn't actually improve your sites rating in the eyes of Google, in moderation of course.
The number of links on the internet that use the nofollow tag is pegged at less than 10% by Matt Cutts. Many webmasters think it's closer to 90% because of what they see when they try to link build but it's not.