Obviously in wiring DC-AC SSRs there is no such things as a common ground (DC on the control side, AC of the load side). DC-DC SSR could have a common ground. But do they need to have a common ground?
It all depends on the application, but by Ground I assume you mean power commons rather than Earth GND.
The terminology is confusing as Ground is used for both, unfortunately.
Max.
One current use of using the same common I am presently using is where a 5v microprocessor is interfacing with a 16vdc system and the micro is using the same supply via a 5v regulator.
A Opto is used to interface the 5v micro with the higher voltage system.
Max.