I have a list of do-follow blogs but they have low PR. If i comment on those blogs by placing my backlinks , will that be beneficial for search engine ranking ?http://s.dpstatic.com/smilies/confused.png
it can be beneficial if your link placed on relevant site and dofollow of course even it low in pagerank, but it would be better if you can find high pagerank page to put your link on.
Anything DoFollow will help you for sure. Just keep in mind that the page from where you get a backlink has some relevant content as your target URL. The more the relevancy, the more the SEO authority.