When a pilot feels the need to jettison their external fuel tanks, do they have to wait until they are above a certain area, or can they just drop them wherever they please?
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
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 thread would indicate that they are actually not dropped all that often.