Most assets are automatically included in asset precompilation. According to the RoR Guide on the Asset Pipeline:
The default matcher for compiling files includes application.js, application.css and all files that do not end in js or css:
[ /\w+\.(?!js|css).+/, /application.(css|js)$/ ]
You would use config.assets.precompile if you have additional assets to include:
config.assets.precompile += ['admin.js', 'admin.css', 'swfObject.js']
Or you could overwrite it.