Thank you for you reply, this is what I was thinking, this is not stored in a dedicated tag stored into the file itself.
My point is that I don't want 5 degrees to rate my songs, having a binary "loved or not" value is much more convenient for me.
Then I still want to keep a 5 stars rating for each "loved" track to keep a trace of this in the file and don't lose it in case I need to export some music.
I think this feature could be easily added to the included Last.FM plugin, I could try to do it by myself but I don't know if the sources are accessible somehere.