In the past I had a small site to which the anchor text to the homepage (index page in your case) was a keyword, and not "home". It worked quite well in terms of ranking on Google. But in my case, the homepage was a long page with content, and it was one of the main pages of the site (small site).
I'm not sure how your homepage looks like. If the homepage just directs users to other pages, then you better try to use your keyword in question on an anchor that sends user to a specific content page, relevant to that specific keyword.
The best and default answer would be - use microformats of
rel="home" inside the anchor tag , and use "home" as the anchor text. This makes navigation clearer for your users, and clear navigation is one of the signals of google for quality.