Here's what I was taught by the "old folks":
Hill your corn when the plants reach about 12 to 18 inches tall (roughly knee-high). Use a wheel hoe or a hand hoe to pull soil from between the rows, mounding it around the base of the stalks. This buries and smothers small weeds, insulates the lower root system, and forces the plant to develop stronger, deeper brace roots. It seems to be common practice around here.
Sowing clumps helps with pollination, support, yield.