This subject seams to pop up from time to time over in the galleries, especially when pictures are posted of girls who pierce low on the lobe. Generally, the best place to pierce is dead center on the lobe. As far as the durability of the piercing, a well healed piercing is amazingly strong even when low to an extent. The big problem comes from a funny trick the body pulls- When skin is irritated, the body produces an enzyme that actually dissolves skin. This is the way the body gets rid of foreign objects. Anybody with piercings should stop wearing them, or go to very small "real gold" studs or hoops at the first sign of any redness. Even a stable long term piercing can cut through in a matter of days or even hours once irritation sets in. You may not even notice it until it's too late. In this case, the higher the piercing, the better the chance you will catch it before it rips through. Conversely, a well healed piercing is almost astonishingly strong. I remember way back in high school commenting to a girl about her big earrings. She proceeded to take a piece of string and hang a book from her earring! (not recommended.)
Of course, there are limits to everything, and if the skin tunnel is already damaged, or irritated, a simple tug may result in a tear-out. Still, there are far more girls out there who have a collection of bent earrings then those unlucky enough to have torn lobes. The best advice is to always keep an eye on your ear and at the first sign of trouble, remove the earrings. If you have multiple piercings, remove the adjacent ones as well. If you did pierce your lobes low but the piercings are well healed, you're probably ok. If it is convenient and looks ok aesthetically, the re-pierce at a higher location when convenient.
