Just a crarification, and personal opinion...
For me when say I dont want to load the PSU over the 60, max 65%, has nothing to do about hitting peak efficiency for lower cost bill. Indeed, the most high quality PSUs have the efficiency curve almost a flat line for the majority of the load range. Most of them even surpass the specs at 50~60% load by 1~2%, but that is not the point. 1~3% or even 5% difference will reflect just a few $$ over entire year. Not worth to think that, unless its something that runs 24/7 with a serious load, like a miner or a server.
The point is elsewhere. Its about maximize life expectancy. I'm taking all possible precautions so I can replace the PSU when I need and/or want, and not when fail. Peak efficiency is cleaner output which also means less stress for the internals. Doing it's job at the most easy point. Also high quality/eff. modern PSUs deal much easier (close to peak efficiency) the ultra high speed transient loads... the way boost clocks are implemented today for modern GPUs/CPUs (Turing, Navi, ZEN2)
I never had to replaced a PSU that has failed.
First quality PSU I bought was 2004, a Enermax Liberty 500W 80+ (100€). Used it for 4 years and left it aside for a 750W PC power & Cooling 82% eff. (140€) that I kept for 11 years with multiple combinations. Last 4 years was loaded with a FX-8370 (since 2012) and a R9 390X (2015), a 450~500W draw, no OC. Still working today with that combo. The Enermax still working today on a PC with 250~300W draw.
I always spend extra for a PSU and never load it past 60~65% and that has worked for me well, for stability (voltage output) and reliability (long life).