Hanging your finished grow upside down is not a must at all. It is simply the best way to pre dry your shit several reasons. None of which is to drain resin to the tops (old wives tale). The main reason is convenience and trichome gland protection and bulbous, capitate preservation.
You see when you hang your buds upside down many of the resin coated single, triple and even quin leaves wrap the mass calyx formations forming a mini bud barrier so to speak. This also makes your buds more aesthetically pleasing in addition to helping protect the long awaited NECTAR!!!