I retired from BA a couple of years ago after 30 years. During that career I flew many different types, using entirely different operating philosophies. After a few years I found, somewhat to my surprise, that it didn't matter too much which operating system was used PROVIDED everybody used the same one.
Latterly BA changed to a standardised system across the fleets, making conversions much easier. Obviously fleet specific procedures override the standard system.
I would imagine that an individual pilot is cast iron legally provided that he/she was following SOPs. An airline might be legally at risk if it were shown that it had modified manufacturer's procedures, but that is not a line pilot's problem.
Unofficial items like “Is it SOP to check the AC meters before putting the generators on-line?” often tend to follow the experiences/prejudices of the chief instructor or technical manager, and are generally not of great importance. On modern aircraft AC will not connect unless volts and frquency are correct - if this is not the case the manufacturer will specify checking. The USOP may reflect the chief trainer's experience from a previous type, and will often have become set in wet concrete as fleet policy.
Different airlines may well operate say the 737NG in very different ways. They all have had to demonstrate to their CAA/FAA etc. their competence to do so. The fact that 737 NGs do not drop out of the sky with great frequency shows that their particular operating philosophy works. In short, there is no "correct" way to operate an aircraft.