If you overdevelop too much you will eventually loose the detail in the highlights. I suggest trying to keep the variable involved in your processing to a minimum and modify developing times according to the final result you want to achieve. The shadow detail, as Thomas Bertilsson said, is basically determined at the time of exposure and you cannot retrieve detail that wasn' t there in first place by developing longer. But you can control the detail of the highlights, to a certain extent, varying the developing time.