OP - I wouldn't worry very much about Nofollow and Dofollow. Just post where relevant and include links when you can. If Google can see where your links are regardless, and see the user who posted it, and they are all Dofollow, don't you think they might already have a method in place to identify the poster as trying to influence page rank?
Google has managed to implement some pretty amazing methods to weed out spammers and pointless content. Identifying people who only post in Dofollow locations wouldn't be much of a stretch.

The key to winning the approval of Google (IMO) is to play by their rules and show a natural progression in your site popularity as much as you can. That involves posting in various places, both relevant and not.

Dofollow which means that Google Juice from other site will pass it onto your site. Nofollow which means it will still gives you links but not Google PR juice.