Is it worth the time to post to free classified ads where you can post a link to your site for a backlink or is a comment on a do follow blog in my topic more valuable in google's estimation?
It doesn't hurt (probably), but it may not help much either. I wouldn't spend much time with it myself. Focus on getting backlinks from pages and/or sites with related content.
i tried this few years back and its a total waste of time. lot of people post to those sites and your post doesn;t stay on the top long enough for the robot to crawl your site. however, if you have some good places to post, might give that a try to see how it works