If it works like the furnace efficiency epic, then you should only need the tax epic active whenever you go to adjust the price of an item in a shop stand or request basket.
But I have no idea if this is how it actually works.
My wild guess would be that it updates each time you enter the beacon with the shop stands because that’s how the tax epic is applied the first time.
That would mean you have to have it in all skill sets or have the right skill set active when you enter the beacon …
This is my assumption too, which were the reason I created this thread. Because if it truly is the case, my current plans will be a pain in the donkey since I only want it on my build spec.