Are your commons wired together?

I am looking for informed opinions.  Here are the pertinent facts:

• 8 power districts 18 VAC wired as buses with 12 AWG wire pairs (red & black); all districts have isolated center rails where they meet other power districts.  No commons are connected, although 1 outside rail is reserved for common.  The other is reserved for insulated rail activation purposes where applicable.
• 3 switch buses 15 VAC wired with 16 AWG wire pair (red, black and white for the DZ-2001 Data Wire Driver).  Each switch bus is powered by a separate K-Line 120F PowerChief transformer;  no commons are connected.

Here are the questions:

1. Should I tie all the power district commons together?
2. Should I tie all the switch bus commons together?
3. Should I tie all of the commons (power district and switch bus) together?

Thanks!

George