Best practice is not to block access to JS / CSS anymore, to allow google to properly understand the website and give determine mobile-friendliness.
Best posts made by J-S
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RE: Robots.txt - blocking JavaScript and CSS, best practice for Magentoposted in Intermediate & Advanced SEO
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RE: How important is admin-ajax.php?posted in Intermediate & Advanced SEO
On a side note, if you don't want hackers to find the wp-admin folder, you may not want to put the folder in the robots.txt file, as this is publicly accessible and it will make it easy to find (although it may not be an issue for automatic attacks).