Find out what rpm the engine develops maximum torque and maximum power - as far as the engine goes, economy and longevity are best if it runs at max torque rpm most of the time - but it needs to get up to max power rpm from time to time.

Theoretically, I've no doubt you are correct.

Many small-ish petrols develop maximum torque at 4000-4600 rpm and are geared 20-21 mph/1000 in fifth . Running most of the time at these revs would mean exceeding NSL or being in third or fourth gear, which doesn't sound either relaxing or economical.

1960's cars typically had max torque around 2500-2700 rpm and were geared about 16mph/1000 so max torque would be 40-ish mph.

I'm well aware that gearing can alter that, hence my comment now in bold

Even though I only do about 3000 - 4000 miles in my car (Mazda3 mk1 1.6 petrol), I do mostly mixed and longer journeys on dual carriageways/motorways, but even so, I still like to 'use the entire rev range' on every month or two (i.e. an 'Italian tune-up) above the normal 2000-3500 revs (70mph in mine is about 3500 revs or so) to clear any gunk from the engine and to ensure, if and when needed, that rev range (and thus power/torque band) is available should I need it for a swift overtake, etc.

I think my car's manual even said I should do so (after the original 'running in period' when new) to a dgree to ensure this, a bit like someone exercising regularly, including both sprints and longer distance running so that their tendons don't get short.

As others have said, labouring the engine, especially up hill and/or loaded up, is never a good idea - modern cars don't mind being revved a bit more than those of yesteryear, especially those like mine with variable valve timing technology or turbos that are specifically designed to get extra performance when needed occasionally by various methods, but essentially by giving it a bit of 'welly' near the top of the rev range. My car definitely feels more 'loose' after me giving it an Italian tune-up.