robots.txt file defines how robots should access folders of your server.The file should stay in root folder.
WordPress can list a single post many ways.On home page,permalink,category wise,author wise.For search engines they are different URLs.So they will give you duplicate content penalty.
So you should write your robots.txt file as follows:
# Disallow all directories and files within important directories
# Disallow all files ending with these unnecessary extensions
# Disallow single post feeds, categories and trackbacks
My current robots.txt file : http://blog.kpsofts.com/robots.txt
Don’t give 777 permission to robots.txt file.Always monitor robots.txt from Google webmaster tools.
Add your upload directory if you don’t want to list your image and other files that you have uploaded.It will block it for image search too.
If you want to add sitemap URL in robots.txt as it as below
Sitemap: your site map file link