On the back I run a slicker smaller tire for increase speed while on the front I run something bigger, usually run it a little softer and with larger knobs for increase steering.

This, for me, is just how I like it. When I lean the bike it hooks up really well. If I am riding xc I use rounder (instead of square profile) tires with smaller knobs. Usually matching as I am not going at speeds where I notice the smallest imperfection in my tires.