That might be a big mistake if you are using dofollow blogs to rank for your keywords most of the time. Dofollow blogs will increase the backlinks for your blog (that is true). It will help to increase pagerank but not helping much in getting higher rank for your keywords (and that it true also). I used to depend on dofollow blog to get my previous blog to PR3 but hardly any traffic coming in.
Just imagine the backlinks from these dofollow blogs: I will get “Bloggertut” as my backlink if I use “Bloggertut” to comment on dofollow blogs. This might help me to rank No.1 for Bloggertut but not the keywords I want to rank for.

Now, moral of the story, giving too much comments on dofollow blog will also cause sudden increases in backlinks. Some blogs have “Top commentor” or “Recent comment” on their sidebars. When you happen to be the top commentor or recent comment that shows on the sidebar, all of the pages on that blog will have a link to your blog. Ha!!! bang!!...you might get an instant 40 or 50 backlinks from this blog and this will get your penalized by Google real fast. I’ve gone into that trap before. Remember that!!! Hope this tip useful for you.