It all sepends on what you want it to do, if your going for uber realistic with cloth sims than you need to model the body underneath as this allows the cloth to collide with the body and make a nice simulaton.

If your not really bothered with making a uber realistic sim then dont bother modeling the body, just model the clothes and then bind then to the character and treat them as you would any skinning job, you could also add blend shapes to add some creases etc for ciritan poses.