As the question implies, how do you determine when it is appropriate to galvanically isolate parts of the same electrical system from one another?
For example, I'm currently constructing a tube amplifier that has a significant amount of digital electronics in it. Currently, the digital part and tube part are powered off of separate windings from the same transformer - However, they are both referenced to the chassis earth (as in each respective circuit has a single wire running from their respective power supply ground to a single point on the chassis - does this even count as galvanically isolated?).
My mongrel amplifier also houses a digitally controlled high voltage regulator section, so I can adjust the voltage to my tubes. For no specific reasons in mind, other than "I don't want my lowly digital signals interfacing with potentially 400 volts!", I decided to stick a cheap isolated DC/DC brick with 1000v internal insulation into the design, as well as a digital isolator, to isolate the onboard DAC from my other digital crap. Attached is an eagle board design view of the offending area, so you can see what I'm ranting about. (sorry, not enough rep for inline images).
The poorly drawn black-dotted line indicates the intended 'isolated' area.
I think my justification for doing this at the time, is that in the likelihood that a component fails on the HV regulator and HV is placed upon any of my poor low voltage components, my digital components on other boards (and there are a LOT of them), would remain safe, rather than be obliterated as the surge propagated through. I don't actually know how true this is, hence the question. Would you consider it appropriate to isolate these two systems in this manner, would you do something different, or would you not even consider isolation necessary at all? In what situations would you consider isolation mandatory?