My understanding is this is more important for aluminum than stainless steel. If you have a new aluminum kettle, just boil water in it for about 30 minutes and that will create a protective oxide layer inside (you'll see the metal turn kind of grayish). It doesn't need to be done regularly, just once when it's new, and again only if you scoured the insides. For stainless, the metal should passivate on its own due to air exposure.
Others may say you do need to passivate stainless, with some claiming you need to use an acid. I've never done it, and never noticed any off-flavors in beers brewed in new ss kettles.