I've read that it used to be very common (in the heyday of straight skis) to bevel base edges by wrapping a turn or two of tape or sandpaper (grit inwards) around a file and using that as a shim against the base to produce the desired bevel on the edge. I've also read that for modern, more highly sidecut skis, this is not recommended and using a base bevel guide is better, because the larger variation in ski width in modern skis will cause the produced bevel to change over the length of the ski.
I can understand that this would happen if you keep the tape or sandpaper shim over the opposite edge, because then the distance from the shim to the edge being beveled will change appreciably as you progress down the ski. But if you hold the file so that the shim is a fixed distance from the edge being beveled (perhaps by using one of your fingers holding the file as a guide against the beveled edge), then the bevel angle would remain constant. The only way it would change is if your base is not flat.
Am I right? Am I missing something?