I would create a CoreBundle, and use this for your template layout, base template, etc.. and do your includes there.
Furthermore if you leave it in the app/Resources making a new directory for Public/css to my knowledge it wont get pulled down when you install/dump your assets to the /Web directory. So if by chance you happen to be only using those files on your live production server, and or like to use that directory as your normal include path, then it just makes more sense.
Also, for me, i use less, and the less files arent compiling and shipping the compiled version into the /web folder either.
If you make a core bundle, you can have all this natively, and have your files in an area that your more likely to be used to getting into with your file editors.
It also helps separate custom files away from the core framework better.
As a sidenote, i did like fsehat's suggestion, however i was a bit disappointed when the assets werent being handled properly. also i dont like using ../ in include paths, ./ is ok, but not ../
if you put them in a bundle (/Your/Bundle/Public/css), after compiling, you can just call them from your templates like this for e.g. <link href="{{ asset('/css/loader.css') }}" rel="stylesheet" media="screen">
