Downloading:
10 different songs from the same host is wrong.
10 different songs from 10 different host if wrong if you are using a 56k modem. If you have a fast connection then it should be ok.

Uploading:
Letting other upload is always good!!! About disconnecting you can set your upload limit to 0, wait till those that already had a upload going are done then disconnect. If they are going to be longer then you can wait you just have to disconnect anyway, after all unlike Napster you can just resume your downloads with Gnutella.