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Follow/nofollow tells Google whether to follow the links on that page, regardless of whether it should index it. Follow is usually safer, because even if it doesn't add the page to Google's index it at least alows Google to follow the links and pass some PageRank around. If a login form is all there is, nofollow would be no problem either, but it can never hurt to have Google follow the links to other sections of your website. The only time I use nofollow in combination with noindex if it's a page that links out a lot (like the almost obligatory 'Friends' or similar linking pages). Those pages I don't want to index AND don't want to pass pagerank through. Anything else: just follow.