While it used to be possible to keyword stuff content to game the search engines, that is less true today. I'm a believer in LSI (latent semantic indexing) whereby the SEs understand the meaning of a page even if they keyword isn't exactly present. SEs today know about synonyms and can extract meaning from text, allowing for creative and interesting use of language. Without tedious keyword repetition the content you create will be of more interest and read better to a search user –increasing the user experience.
I take the viewpoint that you should write for humans first, and then review the content to make sure the keyword is present at least a little bit. I would not go so far as to say you should have 1 occurrence of the keyword per 150 words, or any other hard and fast density metric. But that's just me.
Also, I think the external factors (links with appropriate anchor text, and variations of that anchor text) are more important than on-page factors like keyword density. Just my opinion, although I do rank well for many phrases and I ignore keyword density (other than making sure the keyword is present in some minimal fashion on the page that is optimized for it).