As a recent update in the Webmaster Guidelines Document, Google recommends users not to block the Google ads destination URL with robots.txt file.
In contrast to the previous guideline, the new text in the Technical Guidelines section says,
“Make efforts to ensure that a robots.txt file does not block a destination URL for a Google Ad product. Adding such a block can disable or disadvantage the Ad.”
The previous guideline indicated that efforts should be made to keep the search engine rankings unaffected by the advertisements.Google Makes New Addition to Google Webmaster Guidelines!,