It's not to do with morality, but your torrent software will need to upload (even a little) for your torrents to run at full speed. You see your Transmission needs to share with other people on the internet which parts it has, and request new parts of a file.
Set it to something low (5-10kbps normally works ok). This is just my very basic understanding from what i've experienced in the past (I have to steal work internet as my flat has none ).
However when you get the chance you should try upload a little, if your throttle it, to half your internets ability, you will very rarely notice any speed decrease, even when gaming.
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