Generally, most DJs use the crossfader, although not always. Some genres are easier to mix with the channel faders, for example, house and techno, where you might want to bring up the volume of one track without affecting the other.
However, other genres are more suited to using a crossfader for the faster style transitions - like hip-hop. However, there are no hard or fast rules with crossfader use. It has a different sound and opens different doors compared to channel faders, but both have their uses.