Seeding unfinished torrents only becomes an issue is if your account gets warned for having a low ratio or if you've reached your download cap for simultaneous downloads. While warned, you're only able to seed finished torrents, as incomplete torrents are seen as "downloading" by the system. Likewise, as has been mentioned, they'll count toward the number for torrents you're downloading simultaneously.
Otherwise, it's fine to leave an incomplete torrent open to seed what you have. You just want won't connect to as many leechers as you would if you had all the files.