We have different displays for Terrain. In the Peak mode it is very usefull throughout the flight since you see with one glance how high the highest terrain is below you, very very usefull indeed if you have to get down in a hurry.
Policy is to use Terrain on on both displays from the preflight checklist on, however we are free to switch to radar whenever we like, it is advised to keep one screen in terrain though for departure or arrival, but we are free to deviate from that.