You can't "improve" seek times because the arm is moving at a controlled distance and the disk is spinning at a controlled speed. This synchronization results from a firmly attached arm and disk where centrifugal force has no effect on arm position or disk speed due to friction created by the arm motor, disk hub and bearings. Of couese there might be unbalance caused by an uneven distribution of the mass on the rotational part of the hard disk drive (which was stated earlier by users who noted vibrations can occur at odd angles, i.e. vibration IS unbalance).
Vibration is an important factor limiting the increase of the storage density of all hard disk drives, actually. Some HD manufactures are considering adding an asymmetric spindle motor in order to generate an electromagnetic force which acts against the centrifugal force caused by the unbalance. This is theoretical, only, so for now - stack horizontally or vertically and not at an odd angle.
BTW, some external HD's also have venting, so whatever you do, account for proper ventilation when you stack or position them in a cabinet.
-jim