I was looking for some thing like this, as have few URL's blocked in robots.txt and the ads code is implemented from the header and footer blocks, so the Adsense bot used to give lot of crawler errors. Also google says, it may increase CTR and there by increasing the revenue. Here is the solution I found.
I thought this might useful for some one like me.
Source: here
Code:
# allow adsense bot on entire site
User-agent: Mediapartners-Google*
Disallow:
Allow: /*I thought this might useful for some one like me.
Source: here