If you are behind the saw with the tip pointing away and filing the right-side cutters (i.e. pushing the file from left to right) your left hand (holding the handle of the file) would be lower than your right. It's a pain to maintain at the same time as the 25 degree angle (or whatever) in the other plane. I just sharpened a couple of new chains like that and I can confirm that you need the 10 degree tilt to match the factory sharpening. Whether or not it's important to do so or what purpose the tilt serves, I can't say. After a few more sharpenings mine will probably be close to flat.