Code:
If you have 1 layer board then you dont put Ground Plane on the other side.

If you want fast etching then use as much as possible Ground Plane.

If you want to protect against EMI then use as much as possible Ground Plane.

If you have double sided PCB, then
       If you want all the above then
            use as much as possible Ground Plane.
       Endif
Endif

If you have RF then use as much as possible as much (again) Ground Plane 

If you have double sided PCB, then
       If you want all the above then
            use as much as possible Ground Plane and
            connect with vias as many times as possible.
       Endif
Endif
Conclusion:

Always use as much as possible Ground Plane!

Ioannis