If I can, I always do my underfoot edges on the wet grind belt in the workshop (shoutout Ski Bartlett for letting me do this). Takes 1/100th of the time of filing and does a smoother job. I start at a shallow angle (say 10 degrees) and do 4-5 passes underfoot gradually increasing the angle towards 90 degrees (angle of the base relative to the belt). Then I run my finger along and check for how rounded I've gone. I'll apply less pressure (less rounding) if it's a more all-mountain ski and go pretty much fully rounded if it's a pure park/street ski. Then I use an edge file to detune the contact points lightly. If a ski has early taper, I'll detune everything past the taper point as well, for smoother butters.