Never copper or plastic on anything I'm dealing with. Accident looking for someplace to happen. Maybe it's been OK for 10 years for some, but I've seen enough that weren't OK. A pressure gauge plumbed with -3 will be more responsive than using -4. Bulkhead unions provide a proper pass-through for firewalls, etc. Pressure switches for warning lights, etc are easily installed.

1/16"NPT is not common but it's out there. Schrader pressure valves on some Brand X fuel rails are 1/16 pipe and as mentioned, I've used 1/16" plugs for oil system plugs and restrictors.