Hi–
First off, it's very refreshing to hear questions about "how am I a good contributor to the site?" rather than "how can I get as much out of this as I can?" Ultimately, of course, you get as much out as you put in, but I digress.
Here's what I do for "no leecher" torrents, and why: If a torrent has nobody leeching it, I'll keep it for a few days because someone might come along who hasn't checked into the site recently and want the material. If there are still a large number of other seeders, I might delete it earlier.
If I'm the only person seeding it, I tend to hang on a bit longer, because it I delete the torrent, then it's "dead" on the system and nobody can get it.
As for taxing resources, I don't think having a "no leecher" torrent staying active is a drain on the gt.ru site, but a moderator can weigh in here and correct me as appropriate. As for whether it's taxing your computer, I tend to think not. With a Windows machine you can go to the task manager and see how much memory and processor is being used by your torrent client...most likely, it's not very much, and you have plenty of resources left for other applications. It might be interesting to do a test, and compare the resources before and after deleting a bunch of unused torrents.
One other consideration is whether a torrent is consuming one of your allocated/limited-by-the-system download slots: If you have selected only some of the files in a torrent to download (not all of them), even after those files complete and your torrent client shows 100% completed, the system will still show you as downloading this torrent, because you haven't captured all of it. If you find that you can't start downloading from a new torrent because you've reached your cap, you can fix this by deleting torrents where you've only snatched some of the files. Note that it can take the tracker up to 30 minutes (I think) to recognise that you've deleted the partial torrent and then let you start the new one.
I hope this helps. Welcome to the site, and happy viewing ;D