Generally speaking, the ability to keep the bevels distinct from the flats is a function of your skill with the files and the sanding blocks/jigs mus more than it is a function of the angles.
It's certainly easy to mess up the line, but it's just as easy to get it back. One way to keep it sharp is to put a very slight reverse bevel on the flats. This slightly increases the angle between the primary bevels and the flats, making it more distinctive. It also gives you a way to fine tune the position of the line of demarcation more effectively.