Are there certain areas where a pilot can jettison his/her external fuel tanks?
This depends where in the world you are, over civilian US territory military planes chose to operate under the FAA FAR's (although they are not explicitly legally required to), as such its pretty cut and dry 


  Sec 91.15 Dropping objects.
  
  No pilot in command of a civil aircraft may allow any object to be
  dropped from that aircraft in flight that creates a hazard to persons
  or property. However, this section does not prohibit the dropping of
  any object if reasonable precautions are taken to avoid injury or
  damage to persons or property.


Elsewhere in the world the rules may vary. There may also be rules about dropping tanks over open water due to their being residual fuel in the tanks.

This question covers what happens to them after they are dropped.

This thread would indicate that they are actually not dropped all that often.