It matters in as much that you need to make sure your keywords are in your content, but thanks to the sophistication of Google’s algorithms and their ability to weed out those websites that were spamming search engines using keyword stuffing and other techniques, keyword density is very much part of the “supporting cast” of modern SEO.
You should look to place your keywords in your title tags and other meta data, as well as in content where it would occur naturally.
You don’t need to do what some websites suggest and start calculating your keyword density to the exact percentage!