There is NO risk at all. Even if they were directly pointed to your website (tier 1), your rankings would not go down. Links either have a positive impact or no impact at all in my experience.
What really slowed down my development in ranking sites was all of the fear mongering I read on the internet, and sometimes here. Keyword density this, unnatural backlinks that. For the most part its bullshit- just don't overdo it. The truth is that it's very difficult to negatively impact your site using backlinks. If it was as easy as buying some cheap backlinks or using porn anchors, everyone would be doing it to their competitors
Now, will your rankings improve from shit blog comments/forum profiles? Probably not, but you shouldn't be afraid of them either.