I post songs that I know but not necessarily own. I also must like the song that I post, after all, if I don't like it, how can I expect anyone else to take any pleasure from it.
I also think the song should be obvious from the title/band. Maybe if the lyrics are obviously about the weekly subject it also counts. In general, if you need to explain your choice or quote the lyrics, the song isn't suitable.
As an example, if the subject was "Darkness", Touch too much by AC/DC wouldn't be suitable even if it did say "it was one of those nights where you turn off the lights..."