I have one site that is in a low competition niche and ranks in the top spot for a lot of keywords. That site's anchor text variation is very low, with about 90% of the backlinks targeting the main keyword for each page. However, the off site SEO done to that site is also very gradual and low in volume.
If you are using heavy blackhat techniques to backlink your site, I would suggest that you keep anchor text diversity very conservative. Maybe 50% main keyword, 25% secondary keywords and 25% naked urls, click here, visit this site type of anchor texts.