Amazon S3 for static assets with Rails

Amazon S3 for static assets with Rails

I followed the tutorial from async_assets gem.
It looks ok, but I still don't understand something.
I'll have to ran rake assets:precompile every time I want to update the
assets, right? But if I wan't to ran it only local, without sync? If I
have to run it locally, why I have to put a lot of environment variables
in heroku relating to S3?
It looks for me, looking at logs, that rails somehow get the sass files
instead of compiled CSS files (and also throw some error because of that).
How can I fix that?