From my experience with blog commenting (and I don't have much) - any link that you can get back to your website can help your rankings but you need to focus on high quality backlinks. Nofollow and Dofollow is not as important as it used to be. And, besides Google can tell when you are trying to get too much of one thing. It is better to mix things up a bit and get different kinds of links with different anchor texts, on all kinds of different websites. (i.e blogs, directories, site listings, forums and so on..)
Once you have a diverse link profile, you can start to experiment with slightly more aggressive tactics, but it can be risky. Just make sure you do your research before posting thousands of comments that don't do anything.