Stainless steels can actually rust - like aluminium, their resistance is provided by a thin layer on the surface provided by oxidation of the nickel content. If the pot has been drilled or abraded, this coating is damaged so needs to be restored - this is done by exposing the surface to an acid (in our context, Citric acid is best) in a process called "Pickling"
Basically - dont bother unless you've have the pot extensively mucked about with, soldered or welded 😉
Didn't know that, thanks. I wonder if the brew shop did this with my pre drilled pot...