For optimal rendering and indexing, Google’s new guideline specifies that you should allow Googlebot access to the JavaScript, CSS, and image files that your pages use. This provides you optimal rendering and indexing for your site. Disallowing crawling of Javascript or CSS files in your site’s robots.txt directly harms how well Googles’ algorithms render and index your content and can result in suboptimal rankings.
Read more here - http://googlewebmastercentral.blogspot.co.il/2014/10/updating-our-technical-webmaster.html