I'm not sure why the pros round their backs so extremely - I don't think it's for "shock absorption" as Sheldon suggests, so maybe the racers here will chime in. But I'm guessing it's so they can still pedal when they're in a deep aero tuck. For me, if I'm in even a shallow tuck, with very my shallow drops and high stem, if I'm in neutral spine, my knees hit my chest.

If you're more concerned about the long-term health of your spine, than about your momentary performance, then never force your spine out of neutral for extended periods of time. Some say never intentionally bring your spine out of neutral ever. (I don't necessarily agree, but there are prominent fitness experts who hold that opinion.)

Do use your core muscles to support your back, just as you would sitting or standing. I think the Sheldon Brown piece is talking about people who completely relax their bellies and sink into their shoulders in an extreme hyperlordosis, not people who maintain neutral. The forces of gravity are obviously acting on your spine differently in a tuck than they are standing, but neutral is still neutral, and compressing a disc unevenly is still compressing a disc unevenly.