Hi Sampaul. When i was at the SMX show a few weeks ago there was a whole seminar on keywords. One topic that they went over was keyword density and they totally discredited it. They said that keyword density doesn't matter. I on the other hand believe it does matter. Upon successful optimization campaigns which include writing content the rule of thumb was for every one sentence which contained a keyword or key phrase there should be two padding sentence which don't include keywords. You do not want to be repetitive. The quality of content you offer is much more important then inserting keywords everywhere. Focus on providing your visitors with the best content you can.
In regards to your blogging questions; some blogs can be very beneficial to ones optimization campaign. Make sure you post in blogs that allow the spiders to crawl and index all the posts. Write in detail of your product or services. Then link to your site from the blog using contextual links. Not every blog will offer optimization benefits, some blogs may offer amazing amounts of traffic with no optimization benefits.
If you look around the SERP's in your category, you will almost certainly find a wide array of keyword density readings, from mininmal to quite heavy.
It's not that the engines do or don't pay attention to this. They certainly do analyze keywords on the page. The point is that there are no reliable guidelines and that if you make high quality pages with clear, unique titles and META tags and solid on-page content, that you have put yourself into a position to rank for those pages over time, given enough links to your site, etc.