Both as required. The only thing to really watch for is over compressing your cymbals as this results in the truncation of the tail and the cymbal will only sound for like half the normal time.

vocals require much compression (from a whisper to a scream etc) and it is also used to tighten up drums.

As far as applying compression to the mix, there are two schools of thought or "sounds" those of the east coast and those of the west coast however that is a story for another day.