It is possible to be blocked, but it'll just be for that tracker usually.
It's more out of courtesy. If you can spare the traffic, then it's good to help out.
If you can't, then stop seeding.
Since my upload speed is nowhere near my download speed, I generally seed to about 0.5 ratio. This is more than generous. Others, for example those in the states where upload and download rates are more equivalent, can share to higher ratios.
You`ll find it on mininova, download utorrent to run it. Click on comments on mininova before you download to make sure it`s a good download. When it asks you to open or save, click save and then open with utorrent otherwise Limewire will open it.