frank...I like to cut them off (great in stir frys!). With garlic it is best to cut the flower heads off so energy goes to making the bulbs...seems to me the same for onions. Also, have you noticed that when onions flower they grow a very large hollow stem coming from the center of the bulb? This is the ovule, and tends to have an effect on the growth (and perhaps taste) of the onion bulb. I say cut 'em. As for you grandfather recommending to bend the stalk over, that is usually done when the plant is beginning to yellow a bit, signifying the end of the growth period. By bending over the greens it holds the moisture in the onion bulb but yet starts the curing process.