Here is some instructions given in documentation.
1) It's important that the directory be writable by the webserver user so that Yii can publish the resources there when needed.
2) When a project has multiple versions (production, testing, development, etc.) do not copy the assets/ folders from one area to another; allow Yii to deploy them automatically in each area.
3) Do not manually edit any file under assets/ - if you have a real need to make a change, find the publishing module, edit the source, delete the subfolder under assets/, and let Yii re-publish the updated files.
4) Do not reference names under the assets/ folder directly (say, to get at some other module's assets). If you need to use that
5) Do not add the contents of the assets/ folder to any source-code control system; these files have master source in other places.
6) It is safe to delete everything under assets/. Yii will re-publish the assets if they are not found under assets/.
Hope it will help you :)