Selected answers from the Dumb SEO Questions G+ community.
Ryan Logan: Unless there is a specific reason not to follow a particular link, let ‘er fly. In other words, you’ll mostly want to follow them.
Jim Munro: To add to Ryan`s answer: to conform to guidelines, a link that someone obtains from you commercially should be nofollowed.
Casey Markee: Unless the link is NOT trustworthy or affiliate/sponsored in nature, you should always FOLLOW external links.
Marcus Pentzek: You might want to nofollow links by default if you don`t post them by yourself but let your visitors post it. Example: discussions in your forum or comments in your blog that might contain links posted by your visitors.