Do you mean solder masking ?.. if yes i think solder masking is done everywhere on the PCB where there are no drill holes and places where soldering is not required. its required to protect the PCB from environmental effects.
I mean Green Masking....
Actually i was going through some Power supply where they had done green masking from one side and no masking from the other side
Dont know the Reason and not finding any convincing answer about the same .
sole purpose of Masking is
a. Protect tracks from environmental effects suc as corrosion
b. avoid between two electrical components during soldering, testing.
c. Protects PCB material from environmental effects.
Remember that ESD don´t is affected to varnish used on PCB.
Electromagnetic fields have different behaviour than galvanic fields.
If there are traks and/or plain at both sides, I don´t see any reason to don´t cover that layer.
We said only reasons to use the mask, but didn´t see advantages to don´t use.
Unless if represents cost or time to produce, but don´t think that.