I think this is going around in circles a bit, chaps. A manufacturer provides various means, with appropriate guidance in the use thereof, for the pilot to assess acceptable operational engine performance.
Providing all the bits are working properly, then one ought to do it like the manufacturer suggests, lest one go off at a tangent and be found wanting at the subsequent enquiry.
This is not to suggest that we can't have our own preferences in matters such as the routine use of EPR or N1, where both are provided, so long as we meet all the requirements specified by the manufacturer in the approved flight manual.
I notice that one post referred to the Air Nauru incident. The author of the article is a regular (and very interesting) contributor to Pprune and his expanded tale about the incident makes quite frightening listening. Hence the need for cross-checking those measures which are sensitive to externally caused error.