I agree, keyword density is out since Panda and Penguin. Now it's all about readability and most are avoiding anything that could resemble keyword stuffing like the plague.
As a general rule, and again this depends on your piece and if it flows naturally, you want your keyword in your title and within the first paragraph or two. I use other related phrases people might search for as subheadings and break the articles naturally into nice readable subsections (Google likes that too)
The idea now (thankfully) is actual, good readable content and not the garbage SEO writing and filler from before.