IO540 make an interesting point. If one individual thinks he is on a particular side of a line using GPS and another thinks he is on the other side of the same line using radar, you still have a problem at the time of the perceived infringement.
I don't have an answer to the problem apart from there probably needs to be some form of buffer. ISTR that controllers try to provide some form of lateral buffer inside CAS. Maybe GA pilots should aim to do the same outside of CAS instead of being 'clever' and getting as close as possible to the line (whichever line that is).