I've never tried this, but I think it is a bad idea. You're going to have to remove a lot of material to make a noticeable difference in chainline. Also, by removing material by filing by hand you're not going to get a good interference fit between the spindle and crank when you tighten your crank. Yeah, the 'splines' (corners) of the spindle will stop your crank from spinning relative to the spindle, but it won't seat very well. The odds that your chainrings will remain parallel to the frame are slim. Also, do you really want to remove material from a place under a lot of force?