For people with brand new park skis dulling edges slightly before hitting rails is a good idea. This can be done preferably with a gummy stone, and should probably not be super aggressively done if the skier wants to be able to hold an edge on harder snow. The gummy stone should be ran all along the edge, but particular attention should be paid to segment of edge under the foot. For those who are concerned about taking files to their beat up park ski edges....think about hitting a rail. It's like hitting your skis with a metal pipe. Therefore, if you file down your edges a bit after hitting rails for a while, you probably won't do much more harm, you'll just make the edge wear more even. This will make it harder to catch up on rails.
Sparksnotes: -Gummy stone your new park whips, especially underfoot.
-File down old skis that will be used primarily for rails, to make the wear more even.
That got convoluted, sorry guys.