IF the tip of the soldering iron is either a non-longlife, non-plated type ... or no longer plated due to wear... then filing is indeed necessary. What works best is filing it when heated - this obviously is best not done with an expensive file, it will damage the file eventually - and directly applying solder before a new oxide layer has time to build.
Sometimes, scraping with a thin (disposable/segblade! You will damage a proper edged tool more than you think heating it like that...) knife or screwdriver (thin!, otherwise you conduct too much heat away) immersed in a solder drop gives the most even tinning, since there is no air contact of the bare copper before it wets with tin (this method even works good enough for aluminium to connect a wire to it if you are really patient :) ).
As others have said, this is not the right treatment if the tip is of the plated/longlife type and still intact - once you do it, you will need to frequently do it.
Once you are treating a soldering iron tip as essentially a tinned copper wire, you might want to try hammering the rough tip shape (on hot or cold tip) before reaching for a file: Copper work hardens when hammered, so you get a harder and more durable tip.
Explanation about the "do not use expensive tools": While it is unlikely that a file or knife will wet with electronics grade solder and flux, even less if it is made of stainless steel - chromium alloys are a dog to solder to!, files and knives are heat treated at production time, adjusting hardness, toughness, corrosion resistance. If you heat any of the tool to >>150°C, the heated region of metal will have the metallurgical structure achieved with the heat treatment permanently altered.