Posting in the Magento forums has been disabled pending the implementation of a new and improved forum solution which should better serve the community.
For new questions please post at magento.stackexchange.com, the community-run support site for the Magento community. We will be providing updates on the new forum solution soon. For questions or concerns please email firstname.lastname@example.org.
Is the “magento/downloader” folder needed after installation?
It can be deleted but then the Magento Connect Manager for updating files and install extensions does not work anymore.
Better is to empty the folder downloader/pearlib/download/ ( here are all the files that are installed by Magento Connect Manager and are no longer needed after successful install) also you can clear downloader/pearlib/cache/