50 TO 70 PERCENT LOAD IS OPTIMAL--
The PSU will degrade faster at higher load. That is the fact. A good PSU will work at 90% load, but it will wear out at a faster rate as the components degrade more rapidly. --scryptr
Optimal for what? It's possible to argue that if you won't unpack your PSU it will be even better for its longevity. However personal attachment to the quality of the PSU capacitors and/or whatever internal components may degrade with time should be outweighed by the practical needs and financial reasoning...
Efficiency curve, for starters. Some PSUs can reach up into the next tier of 80+ rating if at 45-65%, and that can sometimes pay for itself in power savings within the warranty period of the PSU. The second is Capacitors hate heat, they'll degrade in higher temperatures than cooler ones, and as they degrade they'll drop your actual max load, so keeping in the upper range of the curve provides both higher efficiency and longer lifespans of the PSU, so you aren't having to pay for a replacement every ~3 years. If it stays cool enough it might still be working for you 8 years down the road, even. May not be the best for immediate start-up costs but will absolutely have its benefits if you stay in the game.