These days, this question is more uncertain than it has been in years.
Traditional opimization and sound keyword density is still important. For reference, you would traditionally see arguments of 3-15+% density depending on page type.
At the same time, a quick read up on stemming, semantics, synonyms, authorities and florida/austin serps will quickly demonstate to you that keyword density is arguably less important than ever, and potentially, a significant detriment to quality rank.
The new smarter algo/filters are about intent and natural, authoritative, and segmented result sets, and kw density is one of the clearest signs that "natural content" is not occuring on your page.
A pragmatic and experienced seo today would likely tell you to write good content, use your keywords phrases in multiple derivations sing/plur, synonyms, etc., think less about density and more about sound content, and *quality links on topic*.