Custom Oak Cycles 650B French Randonneur Project.

but for an audaxer interested in a comfortable ride at a comfortable, non-competetive pace?

http://kingbikefit.com/efficient-cycling-posture-part-1/
"I agree with Joe Friel and the physiotherapists; a posture that maintains a flat back and utilises the gluteals is most effective for power transfer and injury prevention. It is widely accepted that this correct squat technique is the most powerful and safest technique to lift weights, so why not apply this to cycling? In regard to absorbing shock, unless the rider has a problem with the vertebrae itself, bent arms should be sufficient to absorb impact. Using Fabian Cancellara as an example, we can see that he is able to maintain this efficient posture during the 2010 Tour of Flanders. His back is flat and pelvis rotated. He appears to be aerodynamic, comfortable, and powerful. We can also see that he has very well developed gluteal muscles, the most powerful muscles in the body and also partly responsible for knee control. The reason that he is able to maintain this is due to his exceptional flexibility and core strength.

...

It is very easy to explain all the benefits of maintaining a neutral spine, but as you can see with the above examples, there are plenty of extremely successful riders who are not able to adopt this posture. In reality it is very, very hard to train your body to be like Cancellara but you can take steps to improve over time. I believe there are gains to be made in aerodynamics, comfort, and power. These steps can be summarised into 3 main points:

• Improve your core strength and activate your gluteal muscles. Exercises such as squats and physioball routines can quickly transform the way you sit on a bike.
  
• Improve flexibility, focusing on hamstrings and hip flexors. The hamstrings are the main barrier to achieving a neutral spine.
  
• Raise your handlebars, if only temporarily. If your handlebars are too low, it is impossible to maintain a neutral spine and activate your glutes without adopting an extreme saddle position. This is usually a sticky topic, especially among racers. In which case you should consider trying raising your handlebars temporarily to activate those muscles which may not be utilised.
  
  

Significant changes take time, but you should start to feel differences after a relatively short time. Muscles will awaken that you never realised existed.
"
http://coachlevi.com/cycling/proper-road-bike-position-flat-or-rounded-back/

"
But to get to your actual question, you do usually aim to keep your back as flat as possible. On a road bike with your hands in the hooks/drops, your back should be close to flat. However, your flexibility will play a role in how flat you can get your back but still be able to transfer power to the pedals and ride in comfort.
I guess it’s possible some people could ride better with a rounded back, but I can’t see why someone (other than a trained physician) would recommend that. A rounded back is usually poor posture for doing anything. So unless there is some medical issue where it’s a problem to stretch out into a long and low position, I would avoid rounding your back."