That depends on the type of Pedometer you have. Some pedometers use a mechanical pendulum that swings back and forth to track steps. More sophisticated Pedometers like the SC2 Pedometer use a 3D accelerometer which tracks acceleration in multiple directions. Making placement less of an issue.
As a rule of thumb if you have a 2D mechanical arm pedometer you will want to place it mid-waist or on the front of your pants or pocket inline with your knee for the most accurate tracking. 2D Pedometers should also be aligned vertically so the pendulum swings freely and tracks movement correctly.
With a 3D Pedometer like the SC2 Pedometer you will have more flexibility on placement as it measures movement in all directions. As such, it doesn't have to be vertically oriented. 3D Pedometers can be placed in a pocket or bag or alternatively if equipped with a clip can be clipped on your waist, pocket, bra, collar etc. and still track your steps and distance accurately.