Hi,
I agree with barry.
It's not that simple. It depends on a lot of things.
Frequencies, PCB layout, inductive coupling, capacitive coupling, where the voltage drops and where both GNDs connected, where the voltages are referenced to...
The usual steps are:
* first decide the requirements
* then draw the schematic and calculate part values
* decide where on the PCB you want which part of the circuit (connectors, switches, high power part, low noise part, isolation, mounting holes ...)
* then do the part placement
* then do the signal routing
Klaus