I have taught my visitors before on how to properly implement their Robots.txt and just today AdSense just warned their Publisher to check their Robots.txt file in order not to block some Ads, They have explained that Google BOT is different from AdSense BOT so you better add the following codes in your robots.txt file, if you are using Robots.txt for your blog.
User-agent: Mediapartners-Google*
Disallow:
This only means that you are allowing crawl from Google Media Partners. This will also possibly increase relevant ads for your blog.
BTW I have added mine. You can check my Robots.txt file here don’t forget to check my Adsense Tips before, It still help me a lot to get a relevant ads. And I am sure that michigan SEO Consultants will agree with my findings.