Being 19 yet with 15,000 hours of cycling under my belt, i spend almost 50% down in the drops. My average is 3-7k/h faster doing so, and my back is strong and flexible. I spend about 30% on the flat part, which is the next most aero, and around 20% or so on the hoods. Keep in mind i only have one brake on the right side (front) and that plays a role in my preferences. I am riding a fixie right now, though on my road bikes i usually ride in the drops a lot. Keep in mind that i keep my elbows near-locked when doing so.