From Rand at SEOMOZ. I agree – keep it natural…

We’ve talked about how search engines index, examine and measure text – I can assure you that the metric of density is absolutely, 100% unused by any of the major search engines.

And this from way back in 2005

The notion of keyword density values predates all commercial search engines and the Internet and can hardly be considered an IR concept. What is worse, KD plays no role on how commercial search engines process text, index documents or assign weights to terms. Why then many optimizers still believe in KD values? The answer is simple: misinformation.

http://www.miislita.com/fractals/keyword-density-optimization.html