Are using Dofollow blogs to get backlinks to your blog considered a no-no among search engines?

If you are leaving valuable comments on relevant DoFollow blogs, you shouldn't be banned from search engines. Your comment links will be less valuable if they are surrounded by bad links from other commenters (like if someone has no moderation and there are a lot of links in the comments to pharmacies, cialis, porn, etc. Otherwise, there is no reason search engines should ban you for that. The only thing you may consider is if you are doing a heavy amount of commenting using keywords as anchor text as opposed to your name, then maybe mix those keywords up a little so there aren't a ton of links coming into one anchor text. If you are looking for more DoFollow resources, check out the resource listed below. There is a DoFollow blog directory, social bookmarking, forums and more.

No you won't get banned. It's actually a good idea to comment on do follow blogs but make sure your comment is relevant and if you build relationship with that blogger then the blog owner himself will visit your blog.

No banning from Google but its not quite sure that you get benefit from these techniques from PR point of view, rather you may boost traffic to your site.

Not necessarily. The problem is that it will eat up your time, and some marketers just outsource the job. It is also a bit unethical if you just keep making one-time comments and is not really interested in other people's content. The best way to build quality backlinks is to do a massive and relentless article marketing campaign. If you just look, you can find blog networks that actually welcome content from others. In these content - articles - that you can have published in them, you can have your backlinks. Since it's one-way, you have better chances of establishing your site or blog as some sort of authority. It takes time, but it works.