I'll add a few words to what Barney said. The WDH's main purpose is to put weight back on the front wheels. This is for safety because taking weight off the front diminishes your ability to steer and control the tow vehicle. If when the trailer is hooked up the steering feels "squirlley" then there is not enough weight on the front. It should feel the same with the trailer as without.
Generally the front fenders should be brought back to stock height after the hitch is properly adjusted. Most full size trucks and SUVs have a spec in the owners manual for this.
The entire rig need not be perfectly level. Having the trailer lean slightly down is actually desirable.
