For me, I only use around 2% to 3% keyword density. Around 5% to 7% is too much already, and it will cost your content to be suspicious even if it's relevant to the topic.
I don't believe in keyword density just write it naturally. I would concentrate more on keyword prominence and keyword proximity. Just like what I read from the post of benj arriola
With or without keyword density, it's up to you. I agree to the point in writing naturally, but it's just options you know. It's a win-win situation as long your content is unique and well-informative.
keyword density is must for search engine value. But how much % ? Its really unknown but most think it should not be more than 5%.
It should be round about 5-6 %. Write for your readers rather than search engine. Your visitors should be your first priority ... Honey!
I always go with with the phrase " write for user not for search engine". And do so. If user don't write your post what's the use of it? Anyway I never cross 2% of the density. With 2% density your natural and seo friendly as well.
I usually do between 3-6% keyword density but my optimal is 4.5% (or at least that's what I aim for while still making my text look natural!)