personally i only compress after its been recorded.
i think in general, instead of using lotsa compression when i mix id rather try to auto mate my faders to the dynamics of the song, and just use compression where you have to. i know this isnt a popular theory, and its just my opinion, but alota times, it bugs me how compression can become a total crutch for what your mix lacks. plus in alota cases it increases noise.
i dont think compression is bad, its great! i just think its a little abused at times IMO.
take care.