I don't know about other torrent clients, but BitTorrent and uTorrent do save a copy of the .torrent file for every torrent that you've ever imported into them.
I don't know about uTorrent but in BitTorrent, if you tell it to remove and delete the torrent, then it deletes the torrent file from your computer and you would have to re-download the file if you want to seed it again.
uTorrent and BitTorrent are both the same in that regard. The only thing with that is that you must go to "Remove and" then select "delete .torrent" in order to do this. Most users don't bother to do this and will just merely remove the torrent from the list.