The general recommendation would be to seed until you reach at least the 1:1 ratio to give back what you have taken. The BitTorrent network uses a kind of fairness system. In fact, you don’t download from the Mac torrent server, but from other users, so we should seed to be fair with them.
Moreover, the users uploading more to other users are rewarded to a certain extent. The higher seeding ratio you have, the faster download speeds you’ll get. Private torrent trackers can even ban you from downloading if your upload ratio is insufficient. All in all, unwillingness to seed degrades the quality of torrenting. Nobody likes seeing unseeded torrents on the site or experiencing slow downloads.