jonny-that's not true. setting the switches to run m/s on the v3 matrixes the mid signal with the side signal and outputs the M+S/M-S signal. Therefore, you can not re-mix it in post production. once it's matrixed, it's matrixed.
If you have the ability and time to do the post production work, leave the v3 in normal mode and record the mid channel and the side channel separate.