We, as pilots and engineers are bound by rules and regulations. We are expected by the authorities to operate and maintain by the books period.
Yes, but most Pilot operating handbooks and engine operating handbooks are vague where it counts, and are frequently not in full agreement.
Also, most POH / EOH's were written assuming that the aircraft has less instrumentation than is common now.
This is all a bit academic because the only case I can recall where engine operation was an issue is Whyalla. And in this instance CASA were internationally pilloried and nearly 15 years later CASA's reputation has not fully recovered.
If we are called to account in court, we will be required to justify our actions. Adherence to a manual or regulation is the easiest way to do this, but not the only way.