Can't seem to find a definitive answer to this online, many sites say just because SPDS use the ball of foot position, that's not necessarily best for flats
"From a functional movement point of view, trying to place the ball of your foot directly on top of the pedal axle is not the best position for your foot to be whether you are on flats or clipless pedals. Driving through the ball of the foot is what you want to do when you are propelling your center of gravity forward - like when running or jumping - but this is not what is happening when we pedal out bikes. When pedaling you are driving the pedals away from you, much like when you squat or deadlift, and that type of leg drive is much better delivered from a more mid-foot position. This more mid-foot position also allows improved recruitment of the hips during the pedal stroke, especially when standing."
http://www.pinkbike.com/news/Shoe-Pedal ... -2012.html
What do you ride normally? and how do you position your feet on flats?