If you're certain you will be using the bike as a single-speed, using a hub designed for that purpose is superior to cobbling up a single cog on a multi-speed freehub. A hub designed for single speed may have the flanges further apart than a multi-speed hub, making for a strong wheel. Some also accommodate a freewheel on one side and a fixed cog and lockring on the other, making it possible to switch between single freewheel and fixed cog simply by reversing the wheel in the frame.