Connection of the Ground lead on your Princeton Applied Research potentiostat/galvanostat is recommended for the following instances:

1) The ground lead is connected to a Faraday shield surrounding your experimental setup, so that all environmental EMFs that strike the shield will be shunted to ground.

2) If you have a 283 or 263A system, the ground lead is used for Galvanic Corrosion measurements.

Thus, for the majority of applications, the ground lead should not be connected to anything. If you do connect it to something that is already at ground,  you may actually be forming "ground loops" that introduce noise into your system.