I don't use the 'rate source' capability myself. I find it does precisely what I don't want to do, and not what I do want.
I don't really want to rate the source as it applies to the individual facts I am using it to support, which is what 'rate source' gives me (using FTM 2011). I would like to rate the source as a whole, which I can basically do in my own way by commenting in the source citation if need be. More importantly I'd like to be able to rate and justify the fact itself.