Is there any reason to cut the top off the corn stalk after the corn starts growing?

No. The only trimming you want to do is to pull smaller stalks that are within about six inches of bigger ones. Trimming the top risks losing the tassel (the male part of the plant) and then you'll have a tough time getting corn. Corn has to cross-pollinate. That's why you'll get very little corn if you only plant one row. It generally takes four or more rows to get a good harvest, or you can plant them in unorganized groups of six or more plants. The wind, bees, and other insects will carry the pollen from the tassel to the silk (the female part). The silk, which is between the husk and the ear of corn, is actually the flower.