There are multiple schools of thought. The two main ones are:

The Mountaintop school. These users harness up so the base of the dildo is pressing against the pubic mound/Mons Pubis/Mons Veneris. The advantages of this are primarily support-related. Underneath the pubic mound is the pubic bone, so the dildo has a firm foundation for both thrusting and more support in general. Other advantages are that the vulva and clitoris aren't covered by the dildo, which means you could get at them for other activities. The downsides are less stimulation for the wearer from the harness/dildo and less dildo travel for a given length of pelvic thrust (since your pelvis is working like a lever and the dildo is mounted closer to the fulcrum).

The Low Rider school: These users harness up with the dildo sitting over the top of the vulva and clit. Since it's mounted low the dildo travels further with each thrust (great for those who like "long dicking"), and the base rubs up against the clit with each thrust and wiggle. The downsides are a less firm foundation, since there aren't any bones in the immediate area, and some people find it uncomfortable to have the dildo base pressing against their vulva.