Many webmasters (lower case "w" is intentional) seem to think that if one instance of their key phrase 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 over optimized sites. 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.
I personally thing it's a waste of time to dwell on keyword density. Write natural copy and avoid unnecessarily repeating keywords and you should be fine. Things like backlinks count for so much more than keyword density.