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Aerodynamics is all about pushing the ambient air out of your way as you progress along your path.  That air doesn’t know, or care about, the grade of the road — it only cares how fast you are going, which defines how much air has to get out of your way.  A top-tier pro rider may be able to climb a 10% grade at the same speed as a lesser rider would travel on a 2% grade.  Comparing those two circumstances, the benefit to each rider of riding in an aero position will be the same.  So, the only way this question can be answered is to first answer the underlying question, how fast are you going?  Generally speaking, at speeds in the range of 25 to 30 kph, or 15 to 20 mph, there is benefit from improved aerodynamics (riding in an aero position is one of the most impactful of these), and this benefit grows exponentially as the speeds increase above 30kph/20mph.  Of course, the trade-off of aero gain must be weighed against any loss of pedaling efficiency or the rider’s power output.  In most ultra-distance races, there will be a mixture of terrain and almost certainly worthwhile to have the option of at least auxiliary aero ‘clip-on’ bars.  Watching other racers, you’re likely to see that each has a style that they stick to with some consistency; aero tuck above (for example) 30 kph, then switching to a more conventional position such as lever hoods or top of bars as their speed drops, and eventually going all the way to a standing position, drag be damned, when the grade slows them to 10 kph.