Many webmasters (lower case "w" is intentional) seem to think that if one instance of their keyphrase is good, then 500 instances must be better. That was true to some extent several years ago, but not anymore.
Now that the more advanced search engines employ latent semantic indexing, there's simply no benefit to spamming the SE's because the SE's are applying filters to overoptmized sites
In relation to keyword density, this is by far the most intelligent article I've found.
How Search Really Works: The Keyword Density Myth | Search Engine People Blog
Realistically, as long as you don't overdo it, you won't be seen as spamming. Do your level best to write content that sounds natural when read aloud and you'll be fine. Rule of thumb? try to keep your keyword density between 2 and 5% and you should be okay.