A compressor's gain reduction should visually move like an eye blinking.
If you slightly compress every track, bus, and master you should end up with a full sound.
If you need more compression on the master then you're mixing is probably off. If you want a more in your face sound from the master compressor, use 2 compressors in serial. Two compressors back to back that have a gain reduction of -3db will sound a helluva lot better than one compressor that has -6db. -6db is a little drastic IMO.
Also -12db on RMS in general is a good RMS to be at for most genres.