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It's true that subdomains are not internal links. But if they are your subdomains they are trusted links. When you place a link, your PR from that page will automatically flow to it. The only question you have to answer is, now that the link juice has been spent, do you want the link's target to benefit from the link and let your PR to flow to the page? Can you share a context of where or why you think it would be beneficial to use nofollow on a link to one of your subdomains?

Sorry guys, I haven't been clear. The link is to my homepage which is not a design portfolio, but a hosting platform for design portfolios The homepage linked explains the benefits and features you're gonna have when using it to host your web design portfolio.

On the tags and category pages, you should have a rel=noindex, follow tags. That would ensure that crawlers (seomoz custom crawl or Googlebot) dont have duplicate content issues. This was a problem with wordpress blogs as well, which has been handled nicely now.