Keyword density seems to be a minor issue and the key for creating density is to make certain that Google knows what your site is about. From what I have read on SEO forums, you would want to stick to a 3% maximum to avoid any possible penalties. Keyword stuffing will get you in trouble at some point with Google.
You should also be more concerned with developing relevant content and as much of it as possible, that’s what Google and real people really like. If you have a site that has 10% keyword density, it will be about impossible for a real person to read, and that could hurt your conversion rates.