Q:

Low pagerank backlinks - does Google penalize?

I have a new stock discussion forum and I would like to promote it.  Specifically, I have two ideas in mind to help promote it:
1) Become a member at other stock discussion forums.  Make high quality posts, build a good reputation, and leave a link to my own forum in a non intrusive way (ie. in signature or at the end of my posts).  This approach makes sense because you can find other members in other forums that are interested in stock discussion and a backlink to your forum, as long as it is not done in an intrusive/spammy way, should come across as acceptable.
2) Promote my site by writing articles at Squidoo, Hubpages, etc.  This approach also makes sense because that's what Squidoo and Hubpages is for.
The problem with both these approaches is that when I leave a backlink to my site, the page that I am leaving a backlink from may have a low PR - most likely, a PR of 0. Now, I have read that after the Penguin update by Google, your site can be penalized if you have too many backlinks from low PR pages:
http://www.entrepreneur.com/article/224339
So, I am caught in a dilemma:
a) If I start promoting my site via other stock forums, Squidoo, Hubpages, etc, but the backlink to my site comes from a page with low PR, Google may penalize my site.
b) However, if I don't promote my site, nobody will ever discover it (aside from other promotion techniques like social media promotion, directories, etc).

A:

Having a few low PR backlinks does not automatically get you eaten by the Penguin.  Assuming that the forum sites are reputable (and the ones you list are) you should be fine with posting your site as your signature so long as you don't go overboard. If you are posting hundreds or thousands of times per day on multiple boards, you may see some form of pushback as that looks spammy.
One thing to check before you worry is the source code of the boards where you post.  Many sites make their user-generated content pages either noindex and/or nofollow and if that's the case, then you really don't need to worry.
Otherwise, the algorithms are fairly well-tuned at this point and typically don't spike you for "normal" web activity.  If you are just posting your link in your sig and not trying to keyword bomb it, you'll be okay.
tl;dr:
Just behave naturally and you'll be fine.  If you overthink this or overdo it, you'll be nailed.

