It is not very clear if this has any effect on SEO, but as it is very simple to do and it can't hurt if you do it correctly, than I suggest you also pay attention to the keyword density on your posts and pages.

Try to aim for a keyword density of 3-5% but more importantly, make sure the content is readable for the human eye as that would be much more important. With a 3-5% keyword density your content should be very readable to the human eye and would not look spam.

I personally check my posts and pages keyword density with SEOBOOK Keyword Density Analyzer Tool. You may want to consider installing this great SEO toolbar from SEOBOOK as I am sure it will provide you with great information that you must have if you are doing SEO.

You can download the SEOBOOK toolbar using this link: http://tools.seobook.com/seo-toolbar/

Another great SEO Plugin for Firefox is called SEOQUAKE and you can read about it and download it here: http://www.seoquake.com/.

Both plugins will provide you with very important SEO information and will save you a lot of time in your competition research and with anything SEO.