It's more of a personal preference if you want to have it on all the time or if you rather prefer having it being turned on automatically at a certain battery percentage.
I do not have the STAMINA mode activated all the time, since even on the performance based mode, some functions are still disabled. I have access to a charger both at home and at work, so I always make sure that I have enough battery left in the device before leaving work and if needed, I can charge the device when I get home.
If I was going on a trip for a few days and not have access to a charger, I would enable it and keep it on all the time.
There is nothing wrong with always having it and there is nothing wrong with having it always off either, so it's all about personal preference and it will depend on how often you have access to charging the device.